PURPOSE: To improve the intensity of fluorescent X-rays by providing a small window which plays the role of a slit and a window material consisting of a polyester film in a detector.

CONSTITUTION: The fluorescent X-rays are emitted from a sample 1 when the sample 1 is irradiated with the continuous X-rays (white X-rays) generated by an X-ray tube. This fluorescent X-rays pass a 1st slit 2 and are introduced to a spectroscopic crystal 3 and after a wavelength is selected, the X-rays are introduced to a detector (FPC). The crystal 3 and the FPC 5 have a relation of double angles (θ, 2θ) and only the X-rays satisfying the Bragg angle are effectively diffracted and arrive at the FPC 5, by which the X-rays are converted to electric signals. The electric signals are counted by a scaler or rate meter. Al 8 is deposited by evaporation on the polyester 9 to be used as the window material of the FPC 5 to prevent the electrification by the fluorescent X-rays. The resolving power of the X-rays is determined by the slit 2 and the window of the FPC 5 and, therefore, an Ni mesh is omitted by decreasing the window width of the FPC 5 to about 1mm. The resulted X-ray intensity is thus improved.