program writeCSV
  implicit none
  integer, parameter :: n = 5
  integer i
  character linebuf*256
  real :: x(n) = (/ 1.2, 0.02, 0.0003, 4.2, 5.99 /)
  real :: y(n) = (/ 9.0, 0.0008, 0.37, 100000.6, 500.2 /)
  open (18, file='mydata.csv', status='replace')
  do i = 1, n
    write (linebuf, *) x(i), ',', y(i) ! 一旦内部ファイルへ書き出す
    call del_spaces(linebuf)           ! 余分な空白を削除する
    write (18, '(a)') trim(linebuf)    ! 出力する
  end do
  close (18)
contains
  subroutine del_spaces(s)
    character (*), intent (inout) :: s
    character (len=len(s)) tmp
    integer i, j
    j = 1
    do i = 1, len(s)
      if (s(i:i)==' ') cycle
      tmp(j:j) = s(i:i)
      j = j + 1
    end do
    s = tmp(1:j-1)
  end subroutine del_spaces
end program writeCSV