PURPOSE: To eliminate the influence of deviation and eccentricity of a revolving shaft, size tolerance of a holder, etc., during the angle setting of a mirror by fitting the mirror to the revolving shaft through a freely rotatable elastic body.

CONSTITUTION: The revolving shaft 1 is rotated to fix a mirror holding arm 10 at the 1st angle position and the angle of rotation of the shaft 1 at this time does not require high precision. The tips of three adjusting screws 12 are made to abut and a fine adjustment of the mirror angle matching with the 1st optical path 1 is made. At this time, the mirror is attached freely rotatably to the arm 10 through a leaf spring 9, so high-precision angle setting is performed easily. Then, when the mirror is set at the 2nd position, the shaft 1 is rotated by θ° and fixed at the 2nd angle position. Similarly, the three adjusting screws 12' are used to make a fine adjustment of the mirror angle. Thus, this device makes the fine adjustment after setting the arm 10 at the specified position through the rotation of the shaft 1, so the fine adjustment is made by the screws 12 and 12' on the main body side to realize high-precision attachment without the influence of the fitting error of eccentricity of the shaft 1.