PURPOSE: To cancel change in phase due to the change of material or shape if diffraction lattices, and detect positional dislocation with high accuracy by making use of a phase difference between beat signals detected by the use of two light beams which are different in the plane of polarization by 90°.

CONSTITUTION: S polarization with frequency f0 from a light source 1 is divided into two light beams LR and LT by a half mirror 2, they are modulated into lights f1(p) and f2(s) having frequencies f1 and f2 respectively which are fairly different by acoustic light modulators 27a and 27b, and they are then injected to diffraction lattices 15a and 15b over a wafer surface 16 via mirrors 4a and 4b. After that, the (-) primary diffracting light of light f1(p) and the (+) primary diffracting light of light f2(s) are vertically reflected with respect to the wafer 16 so as to be diffracted, they are converged by a lens 6, and they are divided into two lights by a Glan-Thompson prism 7 and an edge mirror 8 thereafter, so that they are then led to sensors 10a and 10b respectively. A first and a second beat signal which are transformed by photoelectric conversion by the sensor 10a and 10b, are inputted into a phase difference meter 11 through a gate circuit 29 which is opened/closed by a clock signal from an oscillator 28, so that the phase difference between both the beat signals is thereby measured.