An optical apparatus for aligning a reticle and a wafer together in connection with reduction projection onto the wafer of an image of a circuit pattern formed on the reticle. Two light beams having slightly different frequencies are concurrently applied to alignment gratings on the reticle and alignment gratings on the wafer through the windows on the reticle and a reduction projection lens. Heterodyne signals of interference rays resulting from diffraction by the alignment gratings on the reticle of the light applied to the alignment gratings are caught by a first optical sensor. Heterodyne signals of interference rays resulting from diffraction by the alignment gratings on the wafer of the light applied to the alignment gratings are caught by a second optical sensor. The difference in phase of the heterodyne signals detected by the respective optical sensors is detected by a phase meter, and the position of the wafer relative to the reticle is adjusted so that the phase difference is reduced to zero.