An X-ray shielding member is provided so as to confront an X-ray incident face of a sample, and a gap through which an X-ray emitted from an X-ray source is passed and irradiated to an X-ray incident face of the sample is formed between the X-ray shielding member and the X-ray incident face of the sample. A gap adjusting mechanism for moving the X-ray shielding member is further provided to move the X-ray shielding member in accordance with change of an X-ray incident angle to the sample by a goniometer, whereby the breadth of the gap formed between the X-ray shielding member and the X-ray incident face of the sample can be adjusted.