PURPOSE: To prevent erroneous measurement by providing a means for discriminating whether the level of diffraction X-rays exceeds a certain reference level or not and taking the result of the discrimination with the level discriminating means into account in the stage of discriminating where thee is an overlap of the peak positions of the level of the diffraction X-rays or not.

CONSTITUTION: The X-rays from an X-ray source 1 pass through an X-ray source shutter 2 and a collimator 3 and irradiate a diamond chip 4. The X-rays transmitted therethrough pass through a slit 6 and enter a detector 7. The chip 4 is rotated by a motor 8 for revolution and an encoder 9 coupled directly to the motor 8 generates angle pulses. The angle pulses pass through an interface 10 and the output from the detector 9 passes through an X-ray counter circuit 11. These pulses and output are inputted to a data processing unit 12 contg. a microcomputer.