An X-ray optical system provides selectively a linear X-ray beam and a point X-ray beam while using an X-ray source which generates an X-ray beam having a linear section. When the point X-ray beam is selected, an X-ray intensity per unit area becomes higher. The X-ray optical system has an X-ray source, a parabolic multilayer mirror to which an aperture slit plate is attached, an optical-path selection slit device, a polycapillary optics and an exit-width restriction slit. The polycapillary optics and the exit-width restriction slit are detachably inserted into a path of a parallel beam coming from the parabolic multilayer mirror, and thus they can be removed from the path and a Soller slit and a divergence slit can be inserted instead.