To accurately detect a defocus quantity on a plane to be inspected from an image plane of a projection optical system with a simplified constitution, under substantially same lighting conditions as those of exposure light.

Exposure light IL projected from a capacitor lens 12 is introduced into a beam splitter 38 through an optical path displacement prism 35, a focus detecting mark 33 on a reticle R is lighted with the exposure light which has passed the beam splitter 38, and a focus detecting mark 31 on a reference member 30 is lighted through a projection optical system PL. The images of the luminous fluxes from the focal point detecting marks 31 and 33 are picked up by an image position detecting system 40 through the beam splitter 38 and an objective optical system 39, and a defocus quantity on the surface of the reference member 30 is obtained from the image forming position of the both mark images.