PURPOSE: To prevent misregistration without deteriorating the fitting accuracy of an optical element, by providing a rib for limiting of movable range of the optical element in a state that a cover is attached to a housing body and the cover is placed on the inside surface of the cover of the housing of an optical system.

CONSTITUTION: On the inside surface of a housing cover 13, a rib 20 is attached to a position where it is limited that a second mirror 6 slides to the right and left, and to the upper part in its mirror surface, when its over is attached to a prescribed position of a housing body 12 and the cover is placed. In such a way, it is limited that the mirror moves to the right and left, and to the upper part in its surface, therefore, for instance, even if a shock is applied due to conveyance, etc., it is prevented that the mirror slides against frictional force between a spring 17 and a reference surface 14, and the mirror, and misregisters remarkably, and it is prevented that the mirror is detached from the fitting reference surface and damaged.