An optical scanning apparatus includes a deflector for deflecting a beam emitted from a light source, the deflector including a rotatable polygonal mirror for reflecting the beam, a motor for rotating the rotatable polygonal mirror, a substrate carrying the motor and provided with a circuit for driving the motor, and a magnetometric sensor carried on the substrate; an optical box accommodating the deflector. The substrate is provided with two holes for fastening the substrate to the optical box. The optical box is provided with two contact portion contacting the substrate in a state that the substrate is fastened to the optical box through the two holes. The magnetometric sensor is disposed at a position closer to a line m connecting the two holes than to a line n connecting two positions on the substrate where the two contact portions contact.