To reduce the amount of air flowing inside a cover.

A rotating polygon mirror 24 is held by a driving motor 30, and the mirror 24 and the motor 30 are covered by the cover 31. An aperture 33 through which laser light beams L1 and L2 are made incident on and emitted from the mirror 24 is provided on the main body part 32 of the cover 31, and a cylindrical part 34 through which the laser light beams L1 and L2 are made incident on and emitted from the aperture 33 is provided on the aperture 33. Control boards 35 and 36 controlling the amount of air flowing inside the part 32 are respectively provided above and below the inside surface of the part 34.