PURPOSE: To improve the flatness of X rays in a 2-D region, by setting spectroscopic crystals on two goniometer heads having rotating shafts perpendicular to each other respectively, and a sample crystal on a goniometer head having a parallel rotating shaft with the second crystal.

CONSTITUTION: An incident X ray 11 is made incident into a spectroscopic crystal 13 set on a goniometer head (GH) rotating about the (y) axis 12 and a diffraction X ray 14 emitted from the crystal 13 is improved in the flatness in the direction (x) while in the direction (y) with a spectroscopic crystal 15 set on a GH rotating about the axis Z. X rays 17 with improved flatness in the 2-D area is incident into a sample crystal 19 set on a GH having a rotating shaft parallel with the crystal 15. The diffracted light 110 thereof is photographed with a nuclear dry plate 111.