PURPOSE: To stabilize a phase difference signal, and to make it possible to conduct an alignment operation in a highly precise manner by a method wherein the diffraction grating pitches of first and second objects are specifically prescribed against the gap G of the title device.

CONSTITUTION: The distance between first and second diffraction grating faces 1 and 2 is set as G, the two wavelengths 3 and 4 emitted from a light source are set as λ1 and λ2, and the lattice pitch P of the first and second lattices 1 and 2 are set in such a manner that they satisfy the relation of the formulas I or II. As a result, the effect of multiple interference is made small, the effect of the amplitude fluctuation of a heat signal becomes small, and the signal treatment by an AGC circuit and the like can be conducted easily. Consequently, a stabilized phase difference signal can be obtained, and a highly precise alignment can be made possible.