subroutine MassFixer( xyr_Press, xyzf_QMix, xyr_PressRef, xyzf_QMixRef, xyzf_DQMixDt )
    !
    ! 成分の質量を補正します. 
    ! *xyzf_QMixRef* が与えられた場合には, 全球積分値が *xyzf_QMixRef* のそれと
    ! 同じになるように補正します. 
    ! *xyzf_QMixRef* が与えられない場合には, 全球積分値が補正前のそれと
    ! 同じになるように補正します. 
    ! *xyzf_DQMixDt* には *xyz_QMix* の変化量が返ります. 
    !
    ! Fix masses of constituents
    ! If *xyzf_QMixRef* is given, the mass is fixed to match its global integrated 
    ! value is the same as that of *xyzf_QMixRef*.
    ! If *xyzf_QMixRef* is not given, the mass is fixed to match its global integrated 
    ! value is the same as that of pre-fixed value. 
    ! Variation of *xyzf_QMix* is returned to *xyz_DQMixDt*. 
    !

    ! モジュール引用 ; USE statements
    !

    ! 物理定数設定
    ! Physical constants settings
    !
    use constants, only: Grav ! $ g $ [m s-2].
                              ! 重力加速度.
                              ! Gravitational acceleration

    ! 積分と平均の操作
    ! Operation for integral and average
    !
    use intavr_operate, only: IntLonLat_xy

    ! 時刻管理
    ! Time control
    !
    use timeset, only: DelTime, TimesetClockStart, TimesetClockStop

    ! 組成に関わる配列の設定
    ! Settings of array for atmospheric composition
    !
    use composition, only: CompositionInqFlagMassFix


    ! 宣言文 ; Declaration statements
    !
    implicit none
    real(DP), intent(in   )          :: xyr_Press   (0:imax-1, 1:jmax, 0:kmax)
                              ! $ \hat{p} $ . 気圧 (半整数レベル). 
                              ! Air pressure (half level)
    real(DP), intent(inout)          :: xyzf_QMix   (0:imax-1, 1:jmax, 1:kmax, 1:ncmax)
                              ! $ q $ .     比湿. Specific humidity
    real(DP), intent(in   ), optional:: xyr_PressRef(0:imax-1, 1:jmax, 0:kmax)
                              ! $ \hat{p} $ . 気圧 (半整数レベル). 
                              ! Air pressure (half level)
    real(DP), intent(in   ), optional:: xyzf_QMixRef(0:imax-1, 1:jmax, 1:kmax, 1:ncmax)
                              ! $ q \Delta p / g $ . 積分値を合わせる層内の成分の質量. 
                              ! Reference specific mass of constituent in a layer
    real(DP), intent(out  ), optional:: xyzf_DQMixDt(0:imax-1, 1:jmax, 1:kmax, 1:ncmax)
                              ! $ \DP{q}{t} $ .  比湿補正率. 
                              ! Specific humidity correction

    ! 作業変数
    ! Work variables
    !
    real(DP):: xyz_QMixBefCor    (0:imax-1, 1:jmax, 1:kmax)
                              ! 修正前の比湿.
                              ! Specific humidity before correction. 
    real(DP):: xyz_DelMass       (0:imax-1, 1:jmax, 1:kmax)
                              ! $ \Delta p / g $
                              ! 
    real(DP):: xyz_DelMassRef    (0:imax-1, 1:jmax, 1:kmax)
                              ! $ \Delta p / g $ of reference
                              ! 
    real(DP):: xyz_DelConsMass   (0:imax-1, 1:jmax, 1:kmax)
                              ! 各層内の成分の質量.
                              ! Mass of constituents in a layer.
    real(DP):: xyz_DelConsMassRef(0:imax-1, 1:jmax, 1:kmax)
                              ! 積分値を合わせる各層内の成分の質量.
                              ! Reference mass of constituents.
    real(DP):: xy_ConsMass          (0:imax-1, 1:jmax)
                              ! 成分のカラム質量.
                              ! Mass of constituents in a layer.
    real(DP):: xy_ConsMassRef       (0:imax-1, 1:jmax)
                              ! 積分値を合わせる成分のカラム質量.
                              ! Reference mass of constituents in a layer.
    real(DP):: ConsMass
                              ! 全球の各成分の質量
                              ! Total mass of constituents
    real(DP):: ConsMassRef
                              ! 積分値を合わせる全球の各成分の質量
                              ! Reference total mass of constituents.
                              !

    integer:: i               ! 経度方向に回る DO ループ用作業変数
                              ! Work variables for DO loop in longitudinal direction
    integer:: j               ! 緯度方向に回る DO ループ用作業変数
                              ! Work variables for DO loop in latitudinal direction
    integer:: k               ! 鉛直方向に回る DO ループ用作業変数
                              ! Work variables for DO loop in vertical direction
    integer:: n               ! 組成方向に回る DO ループ用作業変数
                              ! Work variables for DO loop in dimension of constituents

    ! 実行文 ; Executable statement
    !

    ! 初期化確認
    ! Initialization check
    !
    if ( .not. mass_fixer_inited ) then
      call MessageNotify( 'E', module_name, 'This module has not been initialized.' )
    end if


    ! 計算時間計測開始
    ! Start measurement of computation time
    !
    call TimesetClockStart( module_name )


    ! Check arguments
    !
    if ( present( xyr_PressRef ) .or. present( xyzf_QMixRef ) ) then
      if ( .not. ( present( xyr_PressRef ) .and. present( xyzf_QMixRef ) ) ) then
        call MessageNotify( 'E', module_name, 'If xyr_PressRef or xyzf_QMixRef is given, both have to be given.' )
      end if
    end if


    ! $ \Delta p / g $ の計算
    ! Calculate $ \Delta p / g $
    !
    do k = 1, kmax
      xyz_DelMass(:,:,k) = ( xyr_Press(:,:,k-1) - xyr_Press(:,:,k) ) / Grav
    end do

    if ( present( xyr_PressRef ) ) then
      do k = 1, kmax
        xyz_DelMassRef(:,:,k) = ( xyr_PressRef(:,:,k-1) - xyr_PressRef(:,:,k) ) / Grav
      end do
    end if

    do n = 1, ncmax

      if ( CompositionInqFlagMassFix( n ) ) then

        ! Calculate mass of constituents
        !
        xyz_DelConsMass = xyzf_QMix(:,:,:,n) * xyz_DelMass

        if ( present( xyzf_QMixRef ) ) then
          xyz_DelConsMassRef = xyzf_QMixRef(:,:,:,n) * xyz_DelMassRef
        else
          xyz_DelConsMassRef = xyz_DelConsMass
        end if
        if ( present( xyzf_DQMixDt ) ) then
          xyz_QMixBefCor = xyzf_QMix(:,:,:,n)
        end if


        ! 負の質量を直下の層の質量で埋め合わせ.
        ! Negative mass is removed by filling it with the mass in a layer just below.
        !
        do k = kmax, 2, -1
          do j = 1, jmax
            do i = 0, imax-1
              if ( xyz_DelConsMass(i,j,k) < 0.0_DP ) then
                xyz_DelConsMass(i,j,k-1) = xyz_DelConsMass(i,j,k-1) + xyz_DelConsMass(i,j,k)
                xyz_DelConsMass(i,j,k  ) = 0.0_DP
              end if
            end do
          end do
        end do

        k = 1
        do j = 1, jmax
          do i = 0, imax-1
            if ( xyz_DelConsMass(i,j,k) < 0.0_DP ) then
              xyz_DelConsMass(i,j,k) = 0.0_DP
            end if
          end do
        end do


        ! 全球での補正
        ! Correction in globe
        !   質量保存のために全体の質量を減少させる.
        !   Total mass is decreased to conserve mass. 
        !
        xy_ConsMass    = 0.0d0
        xy_ConsMassRef = 0.0d0
        do k = kmax, 1, -1
          xy_ConsMass    = xy_ConsMass    + xyz_DelConsMass   (:,:,k)
          xy_ConsMassRef = xy_ConsMassRef + xyz_DelConsMassRef(:,:,k)
        end do
        ConsMass    = IntLonLat_xy( xy_ConsMass    )
        ConsMassRef = IntLonLat_xy( xy_ConsMassRef )


        if ( ConsMassRef < 0.0_DP ) then 
          call MessageNotify( 'M', module_name, 'ConsMassRef is negative. ' // 'ConsMassRef is reset to zero, n = %d, ConsMassRef = %f.', i = (/ n /), d = (/ ConsMassRef /) )
          ConsMassRef = 0.0_DP
!!$        call MessageNotify( 'E', module_name, 'ConsMassRef is negative, n = %d.', i = (/ n /) )
        end if
        if ( ConsMass /= 0.0_DP ) then 
          xyz_DelConsMass = ConsMassRef / ConsMass * xyz_DelConsMass
        else
          xyz_DelConsMass = 0.0_DP
        end if

        xyzf_QMix(:,:,:,n) = xyz_DelConsMass / xyz_DelMass

        ! 比湿変化の算出
        ! Calculate specific humidity variance
        !
        if ( present( xyzf_DQMixDt ) ) then
          xyzf_DQMixDt(:,:,:,n) = xyzf_DQMixDt(:,:,:,n) + ( xyzf_QMix(:,:,:,n) - xyz_QMixBefCor ) / ( 2.0_DP * DelTime )
        end if

      else

        ! 比湿変化の算出
        ! Calculate specific humidity variance
        !
        if ( present( xyzf_DQMixDt ) ) then
          xyzf_DQMixDt(:,:,:,n) = 0.0_DP
        end if

      end if

    end do


    do k = 1, kmax
      do j = 1, jmax
        do i = 0, imax-1
          if ( xyzf_QMix(i,j,k,1) < 0.0_DP ) then
            write( 6, * ) 'NEGATIVE: ', i, j, k, xyzf_QMix(i,j,k,1)
          end if
        end do
      end do
    end do


    ! 計算時間計測一時停止
    ! Pause measurement of computation time
    !
    call TimesetClockStop( module_name )

  end subroutine MassFixer

  subroutine MassFixerInit
    !
    ! mass_fixer モジュールの初期化を行います. 
    ! NAMELIST#mass_fixer_nml の読み込みはこの手続きで行われます. 
    !
    ! "mass_fixer" module is initialized. 
    ! "NAMELIST#mass_fixer_nml" is loaded in this procedure. 
    !

    ! モジュール引用 ; USE statements
    !

    ! NAMELIST ファイル入力に関するユーティリティ
    ! Utilities for NAMELIST file input
    !
    use namelist_util, only: namelist_filename, NmlutilMsg

    ! ファイル入出力補助
    ! File I/O support
    !
    use dc_iounit, only: FileOpen

    ! 種別型パラメタ
    ! Kind type parameter
    !
    use dc_types, only: STDOUT ! 標準出力の装置番号. Unit number of standard output

    ! 文字列操作
    ! Character handling
    !
    use dc_string, only: StoA

    ! 宣言文 ; Declaration statements
    !
    implicit none

!!$    integer:: unit_nml        ! NAMELIST ファイルオープン用装置番号. 
!!$                              ! Unit number for NAMELIST file open
!!$    integer:: iostat_nml      ! NAMELIST 読み込み時の IOSTAT. 
!!$                              ! IOSTAT of NAMELIST read

    ! NAMELIST 変数群
    ! NAMELIST group name
    !
!!$    namelist /mass_fixer_nml/
          !
          ! デフォルト値については初期化手続 "mass_fixer#MassFixerInit" 
          ! のソースコードを参照のこと. 
          !
          ! Refer to source codes in the initialization procedure
          ! "mass_fixer#MassFixerInit" for the default values. 
          !

    ! 実行文 ; Executable statement
    !

    if ( mass_fixer_inited ) return


    ! デフォルト値の設定
    ! Default values settings
    !

!!$    ! NAMELIST の読み込み
!!$    ! NAMELIST is input
!!$    !
!!$    if ( trim(namelist_filename) /= '' ) then
!!$      call FileOpen( unit_nml, &          ! (out)
!!$        & namelist_filename, mode = 'r' ) ! (in)
!!$
!!$      rewind( unit_nml )
!!$      read( unit_nml, &           ! (in)
!!$        & nml = mass_fixer_nml, &  ! (out)
!!$        & iostat = iostat_nml )   ! (out)
!!$      close( unit_nml )
!!$
!!$      call NmlutilMsg( iostat_nml, module_name ) ! (in)
!!$    end if

    ! 印字 ; Print
    !
    call MessageNotify( 'M', module_name, '----- Initialization Messages -----' )
    call MessageNotify( 'M', module_name, '-- version = %c', c1 = trim(version) )

    mass_fixer_inited = .true.

  end subroutine MassFixerInit