PURPOSE: To excellently measure superposing accuracy by eliminating an error caused by the inclination of the optical axis of a light optical system.

CONSTITUTION: In the title measuring instrument provided with an irradiating light source, optical system which guides the light from the light source to an object, optical system and camera which take the image of the object, and a picture processing section which processes video signals from the camera, an inverting optical system 13 which relatively inverts the relative relation between a wafer and the irradiating light around the optical axis of the measuring instrument and inverting optical system driving mechanism 14 are provided and an error caused by the inclination of the optical axis is offset from the measured value of the measuring instrument by finding measured values at the relative position between the wafer and irradiating light before and after inversion and averaging the measured values so as to obtain an average measured value.