A plurality of alignment marks disposed on a substrate is photoelectrically detected, and detected photoelectric signals are added to phase-shifted signals produced by shifting the phase of the detection signals to produce a sum signal. The phase is shifted by quantities corresponding to the intervals between the marks to superpose the peaks of the detection signals on the peaks of the phase-shifted signals. A correlation is determined between the sum signal and a reference signal corresponding to the narrow width of detection. A position where the coefficient of correlation is a maximum value is taken as the mark position. Thus, it is possible to use a reference signal having less peaks and a narrow width in the detecting direction, so that the number of times of product-sum calculation for calculation of the correlation can be small as compared with that when a reference signal having as many peaks as the marks and a same periodicity. The mark can be detected in a shorter time to raise the throughput of a mass production of semiconductor devices which are produced by exposing a substrate.