An X-ray optical system for small angle scattering has a parabolic multilayer mirror and, so that switching to other X-ray incident optical systems for X-ray analysis can be easily performed. A parabolic multilayer mirror, an optical-path selecting slit device, a small-angle selecting slit device and a Soller slit are arranged between an X-ray source and a specimen-side slit. An X-ray beam having passed through the first aperture of an aperture slit plate is interrupted by the optical-path selecting slit. An X-ray beam having passed through the second aperture of the aperture slit plate is reflected at the reflecting surface of the multilayer mirror to become a parallel beam. This parallel beam passes through an aperture of the optical-path selecting slit device. The beam width is restricted by a narrow slit of the small-angle selecting slit device.