There is provided a high-performance mirror which is capable of being varied between a plane mirror and a concave mirror. The mirror comprises a base (1), a mirror holder (2), a mirror surface sheet (12) and a back sheet (13) both capable of varying their curvature and retained in a spaced-apart relation by the mirror holder (2) to define an air-tight space between the sheets, the back sheet (13) being deformed from its center to reduce the pressure in the space, the mirror surface sheet being deformed into a concave spherical surface by a difference between the reduced pressure and the atmospheric pressure thereby to vary the curvature thereof.