To obtain a reflector holder capable of reducing a shape change in a reflecting face even when an environmental change is generated and suitable for a highly accurate reflecting optical system by using the same material for a reflector and a base and setting up the center thickness of the reflector to the same thickness as the base.

The reflecting face 1a of the reflector 1 consists of a concave face, a convex face or an aspherical face. The base 2 holds the reflector 1. The reflector 1 is fixed to the base 2 by adhesion or fusion by the use of an adhesive member 3. A base supporting member 4 supports the base 2 and is fixed on a part of a casing. The reflector 1 and the base 2 are constituted of the same material. The linear expansion coefficient of the adhesive member 3 is different from that of the reflector 1 and the base 2. The center thickness of the reflector 1 is almost the same (a difference ≤10%) as the thickness of the base 2. Consequently the reflector holder capable of reducing a shape change in the reflecting face even when an environmental change is generated and suitable for a highly accurate reflecting optical system is obtained.