PURPOSE: To enable a tilt angle of a partial reflecting mirror to be regulated by a biaxial swinging operation easily and rapidly, by providing two tilt angle regulating means between a mirror holder and a body, so that these two means regulate a tilt angle of the mirror holder in different directions from each other.

CONSTITUTION: A distance between a mirror holder 2 and a holding ring 10b secured to a body 1 is held by a tilt angle regulating means having a coil spring 8a and a setscrew 7a. Another set-screw 7b is engaged similarly at a position on a flange 3b rotated by 90° with respect to the setscrew 7a. One end of the coil spring 8b inserted and held at the other end of the holding ring 10b is applied against a part of the flange 3b opposing to the setscrew 7b across a center hole 3a approximately on the same circumference. Accordingly, a distance between the mirror holder 2 and the holding ring 10b secured to the body 1 is held also by a second tilt angle regulating means having the coil spring 8b and the setscrew 7b.