PURPOSE: To precisely and stably measure the superposing precision (shift) of a transferred pattern by using an optical measurement system to detect a rotation error in a pair of diffraction gratings and correct a rotation error in measured light due to disturbance.

CONSTITUTION: To detect the rotating condition of a pair of diffraction gratings (marks) to be measured, the third mark with a small pitch P2 is added to the first layer mark on a wafer. S-polarized light and P-polarized light, somewhat different in frequency, are emitted from a two-frequency straight polarizing laser light source L, injected into a premature prism PB and split into two light fluxes with the reflection of the S-polarized light and the transmission of the P-polarized light. These two light fluxes are injected into the first and second marks on the wafer, refracted, beated with a Glan-Thompson prism GT and then image-formed at points C, D, respectively, so that the pluses can be detected with lock-in amplifiers LA2, LA3 to find a finite difference with a finite difference meter rR. The third mark P2 is image-formed at a point BR and converted into a phase difference equivalent to the amount of rotation. The output of a sensitivity correcting circuit connected to the finite difference meter rR is offset during measuring the shift.