a section near to the rear of the outer circumferential surface of the reflecting mirror 3 contacting a step 2 h formed by the holding holes 2 b and 2 c finally the concave lens 4 is assembled from left to right with respect to fig1 and is fitted in the holding hole 2 a of the fixing member 2 b in addition the inner diameter of this holding hole 2 a is slightly smaller than the outer diameter of the concave lens 4 and hence the concave lens 4 is press fitted in the state of a slight interference fit and fixed in this manner by forming the protrusions 26 26 in the fixing member it is possible to fix the reflecting mirror 3 without fixing means such as adhesive and hence it is possible to shorten assembling time and to reduce cost in addition the reflecting mirror 3 is designed so that it may recover to its original shape after passing through the protrusions with being elastically transformed at the time of assembly furthermore of course it is good to make the protrusions have elasticity instead of making the reflecting mirror 3 have the elasticity in a seventh embodiment of the present invention shown in fig1 holding holes 2 b and 2 c see fig1 adopted in.