PURPOSE: To minimize a reduction in resolution by correcting a focus position judged by an automatic focusing mechanism while taking into consideration a focus deviation from the focus position in an exposure region produced by a spherical aberration.

CONSTITUTION: When a stage 10 is elevated from below and a point A" on a semiconductor substrate 9 coincides with a point A, light from a light emitting element 5 is incident upon a photodetector 8 with maximum luminous intensity and an optimum focus position is obtained. At this time, a vertical directional position ZA from the lower surface of a projection lens 4 to the semiconductor substrate 9 is inputted to an arithmetic unit 15. In contrast to ZA, the vertical directional positions from the lower surface of the projection lens 4 to the semiconductor substrate 9 are obtained by a spherical aberration constant stored in a memory 14 in advance at a plurality of points including outermost peripheral points B and C and the quantity of a focus deviation in an exposure region corresponding to the exposure area of the semiconductor substrate 9 is calculated. The maximum and minimum depths of focus in the exposure region are selected out for the measured focus position by a correcting unit 16 based on the quantity of the focus deviation. A mean value for the two points B and C is obtained, which is made the optimum focus position Z0.