PURPOSE: To make it possible to prevent positional shift of a projected image even if a light transmissive member having different optical path length is inserted into the objective face side of a projection optical system.

CONSTITUTION: Sum of the optical path lengths of light transmissive members 2b, 31, inserted between a projection optical system 4 and a photomask 1, is set constant at all times wherein the projection optical system 4 is set to focus the projection image of a mask pattern 1b on the photomask 1 onto a body 6 to be exposed. Consequently, when a cover glass of different thickness is inserted between the projection optical system 4 and the photomask 1, positional shift of focus image of the mask pattern 1b due to fluctuation of the thickness of the cover glass 2b can be eliminated without adjusting the optical system 4 through combination of the cover glass 2b and another light transmissive which compensates the fluctuation of thickness and making constant the sum of the optical path lengths.