subroutine DIDFT(nmax,nmay,mmax,mmay,a,z)
      real z(0:nmax,0:nmay)
      complex a(-mmax/2:mmax/2,-mmay/2:mmay/2)
      complex gfy(-mmax/2:mmax/2,0:nmay)
      complex i, W, V
      real pi

*-- 定数の定義 ---
      i=(0.0,1.0)
      pi=3.14159
      W=exp(2.0*pi*i/(real(nmax+1)))
      V=exp(2.0*pi*i/(real(nmay+1)))

*-- y 方向の複素 IDFT ---
      do m=-mmax/2,mmax/2
         do n=0,nmay
            gfy(m,n)=a(m,-mmay/2)
            do k=-mmay/2+1,mmay/2
               gfy(m,n)=gfy(m,n)+a(m,k)*(V**(n*k))
            end do
         end do
      end do

*-- x 方向の実 IDFT ---
      do n=0,nmay
         do m=0,nmax
            z(m,n)=gfy(-mmax/2,n)
            do j=-mmax/2+1,mmax/2
              z(m,n)=z(m,n)+gfy(j,n)*(W**(m*j))
            end do
            z(m,n)=real(z(m,n))
         end do
      end do

      return
      end