A casing of an optical scanning apparatus includes: a mounting portion to which a sound-insulating member is mountable; a first and a second support portions supporting a reflective mirror in the casing on which the sound-insulating member is mounted and brought into contact with a back surface of the reflective mirror; and a third and a fourth support portions supporting a reflective mirror in the casing on which the sound-insulating member is not mounted and brought into contact with a back surface of the reflective mirror, wherein the first and the second support portions are positioned between the third and the fourth support portions in a longitudinal direction of the reflective mirror, and contact portions of the third and the fourth support portions protrude toward a reflective surface side of the reflective mirror to be arranged with respect to contact portions of the first and the second support portions.