In the projection exposure apparatus of the present invention, alignment controller selects, from baseline memory, the appropriate baseline amount in correspondence with the insertion, into optical axes and in positional detection optical system, of illumination light limiting member and phase plate. In this way, by using a baseline amount corresponding to a modification of the optical conditions, a desired position on substrate is accurately disposed beneath the projection optical system. Furthermore, even when there are changes in the electrical circuit characteristics of the positional detection optical system as a result of changes in the amplification ratio of the amplifier, the alignment controller conducts processing which is identical to that described above. By means of this, even when the characteristics of the positional detection optical system are changed, the baseline amount corresponding to the changes in characteristics is corrected, and reliable alignment is possible.