Alignment is effected between a mask having a plurality of first alignment marks, a wafer having a plurality of second alignment marks, alignment marks for another step juxtaposed with respect to the second alignment marks and a reference mark indicating the boundary between the second alignment marks and the alignment marks for another step. Alignment is effected by detecting device for detecting the first and second alignment marks and the reference mark and putting out a signal stream, and an electrical circuit for effecting the introduction of signals from the signal stream in accordance with a predetermined timing, extracting from the signal stream a signal regarding the reference mark and effecting the introduction of signals from the again detected signal stream at a timing changed in accordance with the signal regarding the reference mark.