A frame 18 with a first lens group 15 and a second lens group 17 of a projection lens system fixedly mounted thereto is supported by a support arm 43 so as to direct a mirror mounting aperture 27 of the frame 18 downward. The reflection mirror 16 for folding an optical axis of the projection lens system is held and placed within the mirror mounting aperture 27 of the frame 18 by a mirror support 46 so as thereby to provisionally complete the projection lens system. Then, while an image of a test pattern generated by a test pattern projection head 44 is projected onto a remote screen 47 by the provisionally complete the projection lens system, the reflection mirror 16 is adjusted in position by 45 by a mirror position adjusting unit 45. When the test pattern image on the screen is correctly projected, a bonding adhesive is filled in a circumferential clearance left between the reflection mirror 16 and the mirror mounting aperture 27 and cured by use of an ultraviolet lamp 49 to bond and fix the reflection mirror 16 to the frame 18.