PURPOSE: To obtain a filter device for an X-ray source capable of exchanging the filter easily without opening a vacuum vessel to the air.

CONSTITUTION: A filter device for an X-ray source has a filter 11a contained in a vacuum vessel together with a laser plasma X-ray source 1, penetrating X-rays 4 discharged out of the laser plasma X-ray source 1 and preventing the penetration of other scattering particles than X-rays. Filters 11a, 11b and 11c are arranged in at least two regions so that when one of them is inserted in an X-ray 4 path, the other is out of the X-ray path. The device also has a drive means 12 to drive to expel the region inserted in the path out of the path and drive to insert the region 116 expelled out of the path into the path.