PURPOSE: To measure a curvature of a reflection surface such as a mirror by transmitting an electromagnetic radiation beam to a sample reflection surface, and measuring an angle deflection of a reflection element which makes the reflection radiation converge into a detector.

CONSTITUTION: On a platform 8, a detector 9 and a lens 10 which sends, to the detector 9, radiation beam 11 reflected on a lens 4 where the curvature of a surface 5 is measured, are provide. In addition, a mirror (reflection element) 12 is so attached that it is rotated by a mirror rotation means 13 around an axis 14. A means 13 generates a signal representing an orientation of the mirror 12. The signal is, together with the signal which the detector 9 generates, applied to an analogue/digital converter 16. Change in orientation of the mirror 12 which is required for the beam 11 to be incident on the detector 9 is related to an angle θ between an incident beam 7 an the reflection beam 11, and it is decided by a processor 18. By knowing the angle θ and a movement distance of a platform 1, the curvature of the surface 5 of the lens 4 is calculated.