PURPOSE: To enable a precise printing of a very fine mask pattern on a semiconductor wafer, by obtaining an X-ray highly parallel and of single wave length utilizing two times Bragg's reflections in an optical system of an X-ray stepper.

CONSTITUTION: A parallel flat plates and double crystals monochrometer 1-1 has double reflection surfaces 1-4 and 1-4 which are cut out from a single crystal, and these reflection surfaces face parallelly each other and also are placed at a Bragg's reflection angle θB to an incident X-ray 1-2. The Bragg's reflection angle θB is calculated from a distance between the two crystal surfaces and wave lengths of incident and emitting X-rays. In this way, the incident X-ray 1-2 is emitted as a highly parallel monochrome X-ray 1-3 after repeating the Bragg's reflections twice, and because the directions of incident and emitting rays are the same, a mask at the tip end of an optical system can be easily prepared.