PURPOSE: To detect how much the line-shaped mark of an object to be measured deviates for a specific reference position.

CONSTITUTION: An object 40 to be measured has a symmetrical line-shaped mark 40a. A position deviation operation circuit 17 is added to a light scanning type displacement meter 16. The light scanning type displacement meter 16 scans light beams a for an object 40 to be measured while being crossing the line-shaped mark 40a by a scanning mirror 5, obtains signals V1 and V2 corresponding to an incidence position by applying reflection light to a position detection element 8, and then obtains a displacement amount signal L=(V1- V2)/(V1+V2) corresponding to the displacement of a distance Lb from the object 40 to be measured by operation. The position deviation operation circuit 17 judges whether the displacement data of the line-shaped mark 40a is symmetrical or not based on the displacement amount signal L and then outputs a position deviation signal Sp with a sign and size corresponding to the degree of asymmetry.