PURPOSE: To accurately detect the two-dimensional position of an object to be detected, by correcting astigmatism generated in an optical system for position detection.

CONSTITUTION: The pattern of a reticle R is exposed on a wafer W via a projection optical system. In an alignment system 4 of the side surface of the projection optical system, an alignment light from a wafer mark WM on the wafer W passes an objective 7, a beam splitter 6, an elliptical zone plate 8, and an elliptical zone plate 9, and forms an image of the wafer mark WM on a focal plate 11. The image on the focal plate 11 is picked up by image sensing elements 13M and 13S via an imagery lens 11. By changing the absolute rotation angles or the relative rotation angles or the interval of the eliptical zone plates 8 and 9, astigmatism generated by the objective 7 and the beam splitter 6 is chancelled.