PURPOSE: To make it possible to quickly conduct the focussing operation of a projection optical system in a highly precise manner by a method wherein a slit member, having an inclined slit-like aperture part, is provided on the position optically conjugated to the first object surface, and the image-formation state of the slit-like aperture part on the first and the second object surfaces is detected.

CONSTITUTION: The light flux emitted from a light source part 1 passes through a capacitor lens 2, a part of which passes through a beam splitter 3 and illuminates a slit member. The light flux, which has passed through the slit-like aperture of the slit member 4, is image-formed on the pattern surface 7b on the backside of a reticle 7 by the image-forming lens 5. At this point, the slit member 4 is inclined to an optical axis La. The slit image is passed through the slit-like aperture of the slit member 4, reflected by the beam splitter 3, and image-formed by an image-forming lens 11 on a detector 13 by changing the direction of light flux by a reflection mirror 12, and the positional deviation of the lower surface of the reticle and the detector can be detected.