PURPOSE: To provide an apparatus for measuring a phase difference even in the phase region of a halt tone mask appropriately.

CONSTITUTION: A luminous flux (a) passed through a polarizer 13 having the plane of polarization rotatable freely through a control unit is separated in two through a crystal element 15 and a condenser lens 16 and projected toward a photomask 8 having a transparent part and a halt tone part 9. Relative intensity of the fluxes (b), (c) can be varied through rotation of the polarizer 13. When the polarizer 13 is rotated according to the ratio of transmittance between the transparent part and the half tone part 9 to produce fluxes having different optical intensity, for example, two fluxes passed through the photomask 8 have substantially equal optical intensity and a luminous flux, recombined through an objective lens 20 and a crystal element 21, can attain a sufficient contrast. When the phase difference is detected by observing an interference image, the contrast of interference intensity is prevented from lowering and the interference fringe is prevented from being buried in one light. Consequently, phase difference can be measured conveniently even if the half tone is not an objected to be measured.