PURPOSE: To expose wafers by always projecting a mask pattern upon the wafers at a set projecting magnification by changing and correcting the projecting magnification of an optical system based on the error of the projecting magnification against the set magnification.

CONSTITUTION: A mask 2 is positioned through an optical system 3 by using reference marks 6 on a wafer stage 4. After positioning the mask 2, an arithmetic system calculates the theoretical projecting position of each mark 7 for measurement through the optical system 3 based on a set projecting magnification. Then the marks 9 for measurement of the reference marks 6 on the stage 5 are moved and positioned to the calculated theoretical projecting positions of the marks 7. After positioning the marks 9, both the marks 7 on the mask 2 and the marks 9 for measurement of the reference marks 6 on the stage 4 through the optical system 3 are simultaneously viewed through an optical microscope 8 for measuring projecting magnification. Based on the deviations between all marks 7 and 9, the arithmetic system approximately calculates the error of the projecting magnification against the set magnification and corrects the error.