The EUV light source device eliminates radiation other than EUV radiation from the light which it emits, and supplies only the EUV radiation to an exposure device. A composite layer consisting of a plurality of Mo/Si pair layers is provided upon the front surface of an EUV collector mirror, and blazed grooves are formed in this composite layer. Radiation emitted from a plasma is incident upon this EUV collector mirror, and is reflected or diffracted. The reflected EUV radiation (including diffracted EUV) proceeds towards an intermediate focal point IF. The radiation of other wavelengths proceeds towards some position other than this focal point IF, because its reflection angle or diffraction angle is different. A SPF shield having an aperture portion is provided at the focal point IF. Accordingly, only the EUV radiation passes through the aperture portion and is supplied to the exposure device, while the other radiation is intercepted by the shield.