PURPOSE: To reduce a noise in an X-ray measurement using a sample chamber and to obtain a high S/N ratio by a method wherein an indoor screen compris ing an opening part through which X-rays are passed is installed between an incident window for the sample chamber mounted on a goniometer and a sample and the opening part is moved in parallel as the goniometer is turned.

CONSTITUTION: An incident window 3 and a radiant window 4 are attached to a main body 1, and an indoor screen 7 through which incident X-rays 8 are passed is installed between the incident window 3 and a sample 5. The outer size of the screen 7 is made larger than the incident window 3 as far as possible, and it is required to reduce a distance up to the sample in order to reduce a noise. A sample chamber is mounted on a goniometer 13 which can be turned. Since the rotation center of the goniometer agrees with the center of the sample, the position of the opening part is changed as the sample chamber is turned. Then, a signal is sent to a driving gear 15 for the screen 7 by a control device 16 which controls the goniometer 13, the screen 7 is moved in parallel via a driving shaft 14, and the incident X-rays 8 can be always passed through the opening part.