To realize highly accurate alignment by suppressing the effect of measurement error due to variation of base line and detecting the position of an article to be detected (position detecting mark) with high accuracy.

The aligner comprises an illumination optical system 3 for irradiating an original plate, i.e., a mask 4, with illumination light from a light source 1, a projection optical system 7 for forming a pattern image formed on the mask 4 on a photosensitive substrate, i.e., a wafer 8, and an alignment sensor 16 constituting a position detecting system for detecting an alignment mark on the wafer 8. A pattern region on the wafer 8 is formed at a position shifted to the alignment sensor 16 from the projection center of the projection optical system 7 and the alignment sensor 16 is disposed on the side close to the pattern region formed on the wafer 8 while being shifted from the optical axis AX.