Abstract

An optical element comprised of a plurality of polarizing beam splitters is provided on the optical path of a two-frequency linearly polarized laser beam source, and the optical element divides a light beam from the two-frequency linearly polarized laser beam source into two light beams, whereafter these two light beams are incident on diffraction gratings provided on a mask and a wafer, respectively. A pair of mirrors are provided vertically above the mask, and a pair of lenses, a pair of polarizing plates and a pair of photoelectric detectors are arranged in succession in the directions of reflection of the pair of mirrors. The outputs of the two photoelectric detectors become beat signals, and the phase difference between these beat signals is measured, whereby the alignment of the mask and wafer is effected. Since the optical element is comprised of a plurality of polarizing beam splitters, leak-in light included in each light beam is reduced.