to the synthesizer 45 after passing through optical axis tuning units 46 and 47 and beam benders 48 and 49 since the monitor light l 0 enters the synthesizer 45 as an s polarized component it is reflected by the synthesizer and directed towards a half wavelength plate 50 on the other hand the pattern drawing beams l 1 and l 2 which enter the synthesizer 45 as p polarized components are first passed through the synthesizer 45 and then directed towards the half wavelength plate 50 the lens units 35 and 42 form a beam expander which enlarges the incident pattern drawing beams l 1 and l 2 so that the diameter of the beams emerging from the lens unit 42 will be 1 67 times as large as the incident beams the direction of polarization of the pattern drawing beams l 1 and l 2 and the monitor light l 0 is rotated by the half wavelength plate 50 through 90 u00b0 they thereafter are passed through a lens unit 51 and a beam bender 52 directed to a lens unit 53 the pattern drawing beams l 1 and l 2 and the monitor light l 0 that have passed through the lens unit 53 are deflected towards the polygonal mirror 1 by way of beam benders 54 55 and 56 and reflected towards the surface of the workpiece 4 on the drawing board 15 by means of a surface segment 2 of the polygonal mirror 1 the lens units 51 and 53 function as a beam expander that enlarges the spots of the pattern drawing beams l 1 and l 2 and the monitor light l 0 to diameters about 21 times as large.