A guide convex portion is formed on a base plate in parallel to an illumination optical axis. Further, a fixed-side supporting member is provided in the base plate. A first opening is formed in an upright portion of the fixed-side supporting member. A slide supporting member can be provided on the base plate. A second opening is formed in the upright portion of the slide supporting member. By inserting a tapered rod into the second opening at its light incidence side and by moving the slide supporting member along the aforementioned guide convex portion, the slide supporting member is stopped at a predetermined position and, at this position, the slide supporting member is fixed.