PURPOSE: To measure the crystal azimuth checking of twin by rotating a sample crystal being irradiated with an X ray only on the rotating shaft in such a manner as to be at a low speed near the area where an X ray diffracted is detected to meat Bragg conditions for a specific crystal face.

CONSTITUTION: An X-ray of an X-ray source 1 irradiates a diamond chip placed and joined at the tip of a shank shaft 3 through a collimator 2. The chip 4 is rotated on the shaft 3 and the diffraction of the X-ray (d) takes place only at such an angle that Bragg conditions are met between the crystal face and an incident X-ray (a). Only those X-rays diffracted with a specified face index are selected with a slit 6 and detected with an X-ray detector 7. A sample carrier combined microswitch 9, an X ray source shutter 10, a sample rotation motor 11 and an encoder 12 for generating an angle pulse are connected to a data processor 15. The chip 4 is rotated at a low speed near the angle area where X-ray is expected to be detected with a detector 7 and at a high speed in other areas. The presence of the twin and the crystal orientation can be indicated quickly on a CRT 16 or the like.