Disclosed is a projection exposure method for transferring a pattern formed on a mask onto a photosensitive substrate through a projection optical system. A light beam having a first wavelength for exposure is radiated through the projection optical system onto a first mark area including a fiducial mark on a fiducial plate installed on a substrate stage, reflected light from the first mark area is detected to obtain a position of the fiducial mark. A light beam having a second wavelength to which the photosensitive substrate is not photosensitive is radiated through the projection optical system onto the first mark area, reflected light from the first mark area is detected to obtain a position of the fiducial mark. A positional discrepancy of the fiducial mark caused by the difference in wavelength between the first and second wavelengths is previously calculated on the basis of results of the detection. The light beam having the second wavelength is radiated through the projection optical system onto an alignment mark on the photosensitive substrate, reflected light therefrom is detected to obtain a position of the photosensitive substrate under the light beam having the second wavelength. A positional discrepancy of the photosensitive substrate is corrected on the basis of a result of the detection and the calculation, and thus positional alignment for the photosensitive substrate is performed, followed by actual exposure.