PURPOSE: To obtain a frequency selective sub-reflecting mirror whose structure is simple and whose loss is small by constituting the sub-reflecting mirror of a metallic specular surface, a frequency selective metallic filter and each one layer or a dielectric sheet layer and a dielectric core layer.

CONSTITUTION: A ray 6a passes through a frequency selective metallic filter 22a, a dielectric sheet layer 23a and a dielectric core layer 24a of a sub- reflecting mirror, reflected by a metallic specular surface 21, passes through the core layer 24a, the sheet layer 23a and the filter 22a again and goes to a main reflecting mirror. In this case, by making thickness of the dielectric core layer 24a constant, and changing thickness of the dielectric sheet layer 23a, adjustment of a phase is executed. As for thickness 3b, the larger an incident angle θ to the sub-reflecting mirror of the ray 6a is, the thicker it is, and the smaller θ becomes, the thinner it becomes. In such a way, the number of components is small and the equipment can be manufactured easily, and also, by adjusting the thickness of the dielectric sheet layer, the loss of a passing ray is suppressed.