To effectively reduce the scattered particles by mounting a scattered particle blocking member in parallel to or approximately parallel to a face formed by optical paths on optical paths of incident light to an X-ray reflection optical element and/or the light reflected from the element.

A scattered particle blocking member 8 made of an aluminum thin plate is mounted in parallel to a face formed by an optical path of X-ray entering into a reflection optical element 7 and a parallel beam bundle 13 of the X-ray reflected by the reflection optical element 7. By mounting the scattered particle blocking member 8 on this place, the optical paths for the X-ray entering into the reflection optical element 7 and the X-ray reflected thereby are hardly intercepted. That is, the optical paths of the X-ray is not affected, and the attachment of the scattered particles scattered by a buffer gas to the reflection optical element 7 can be prevented. The thickness of the scattered particle blocking member 8 is preferably as thin as possible so as not to intercept the X-ray and actually below 2 mm.