To correctly measure a shape of double faces and a thickness irregularity when interference fringes are measured with an insufficient parallelism of reference faces of a pair of interferometers by constituting an apparatus for measuring the shape of double faces of parallel flat plates and the thickness irregularity with the use of the pair of interferometers to be able to measure a parallelism of the reference faces when interference fringes are measured.

A member 14 for holding a body to be tested is set retractably outside optical paths of interferometers 12A and 12B. A shading member 30 which can shield a coherent light from a light source of the interferometer 12B is set to the interferometer. Before or after interference fringes are measured, the holding member 14 for the body to be tested is retracted outside the optical paths, and moreover the coherent light from the light source of the interferometer 12B is shielded by the shading member 30. In this state, interference fringes between reference faces 22Aa and 22Ba are measured by the interferometer 12A. When a surface shape and a thickness irregularity of faces 2a and 2b of a parallel flat plate 2 are to be measured, a deviation in parallelism of the reference faces 22Aa and 22Ba can be corrected in this manner.