which has entered the x stage 40 through the introduction port 50 has its beam diameter stopped down by a stop 70 provided on the optical path thereof in this case the beam diameter of the laser beam l is made sufficiently larger than the diameter of the stop 70 whereby even if the x stage 40 moves finely in z direction the laser beam l may not deviate from the aperture of the stop 70 subsequently the laser beam l which has passed through the stop 70 has its beam diameter adjusted by a beam expander optical system 80 the laser beam l is then reflected by a mirror 90 travels in a direction parallel to the z axis and illuminates the pattern 100a of a regulation plate 100 as the reference member of the present invention provided on the x stage 40 subsequently the laser beam l transmitted through the pattern 100a of the regulation plate 100 reaches a mask m through a projection lens 110 and forms the projected image of the pattern 100a of the regulation plate 100 on the pattern surface of the mask m further the laser beam l transmitted through the mask m is received by a photodetector 140 via a mirror 120 and a condensing lens 130 a circuit pattern mp to be exposed onto a wafer w is formed on the mask m the wafer w is held by a wafer holder 150 provided on the x stage 40 the reference numerals 160 and 170 designate reflecting mirrors for measuring the positions of the x stage 40 in x direction and y direction by the.