A filter capable of adjusting spectrum in multiple stages and that capable of attaining the reduction of size, as well as an X-ray imaging system having such a filter, are provided. The filter, which is for adjusting the spectrum of passing radiation, comprises a support plate having an aperture for passage therethrough of radiation, plural filter plates supported by the filter plate and having mutually different filter characteristics, and moving device for moving the plural filter plates selectively to a position to close the aperture and a position to open the aperture.