To widen an inclination adjustment range and to simplify the adjustment.

A full reflection mirror 12 is supported at three places in total which are at almost the center of a lower part of the full reflection mirror 12 and the left and right parts of an upper part of the full reflection mirror 12 by making the outer circumferential surfaces of disks of eccentric screws 24a and 25a provided eccentrically with the disks 24b and 25b at a point part that is screwed into each screw hole 23b provided on the bottom and top surfaces of a mirror holding frame 23 abut on one side surface of the full reflection mirror 12 with a line. Then, by turning the eccentric screws 24a and 25a, the full reflection mirror 12, on the other side surface of which one end sides of springs 26 and 27, the other end sides of which are fixed to the mirror holding frame 23, are allowed to be abut is moved in each supporting place back and forth in the direction an optical axis Lo.