To constitute a mirror unit, capable of excellently performing action to make scanning light incident or exit by providing two counter mirrors and a movable mirror arranged in the mirror unit in an enclosed state and constituting a counter mirror base plate which supports the counter mirror with a light- transmitting member.

In the mirror unit 1 arranged in the optical scanner, the movable mirror 4 is held on a movable mirror base plate 2, so as to reflect irradiating light several times with the counter mirror 3. The counter mirror member 10 holding the mirror 3 is constituted of the light-transmitting member, so that the light is made incident or made to exit, without using an aperture or the like. In the case of assembling two base plates, the mirror is accurately positioned, and assembly work is easily performed, by providing an alignment mark 9 on either base plate.