To prevent the slippage of the emitting direction of measuring light or the optical axis of a focusing lens system from affecting on measured results even when the slippage occurs due to the unstable attitude of a light source.

After the measuring light emitted from the light source 11 is split into first and second light rays, only the first light ray is condensed to a prescribed incident optical axis of an object 91 to be inspected and the first light ray reflected by the surface to be inspected of the object 91 and the second light ray which does not reach the object 91 are respectively caused to form light spots by introducing the light rays to the same measuring surface. The slippage of the centroidal position of the total light quantity of both light spots from a reference position is detected by means of an optical position detecting element 70 and the eccentricity of the object 91 is calculated based on the detected slippage. At the time of splitting the measuring light, the light quantities of the first and second light rays are adjusted to become almost equal to each other. In addition, the light rays are introduced to the measuring surface, so that the first light ray may slip by the same amount as that of the measuring light which slips in the emitting direction in the same emitting direction, and the second light ray may slip by the same amount in the direction opposite to the slipping direction of the first light ray.