Only X-rays having a specific wavelength, selected from a group of focusing X-rays diffracted from a sample, are reflected from a monochromator based on a Bragg's condition, passed through a receiving slit and detected by an X-ray detector. The monochromator is configured to be freely removable, and arranged between the sample and a focal point at which the wavelength-selected focusing X-rays diffracted from the sample are directly focused. At this time, the monochromator is moved so as to position the monochromator as close to the focal point as possible. The monochromator comprises a multilayer mirror having an internal interplanar spacing, wherein said internal interplanar spacing varies continuously from one end of the monochromator to the other end.