In an EUV focusing mirror in which there are several thin concave nested high precision mirrors in the form of an ellipsoid of revolution, a paraboloid of revolution, a Wolter type or the like, the sides which do not constitute the reflection surfaces are made in the shape of a knife edge at the radiation incident ends of the respective mirrors in order not to be shielded by the thickness of the radiation incidence sides of the respective mirrors. Likewise, the radiation exit ends of the respective mirrors are made in the form of a knife edge. This yields an advantageous far-field distribution use of the mirrors for an EUV radiation source device and the degree of reduction of the light intensity can be made smaller than in the conventional case.