The light beam deflection cycle of a rotational polygonal mirror includes first and third sections in which a latent image is not formed, and a second section in which the latent image is formed. A driving signal supply unit supplies a driving signal of a frequency less than the resonance frequency of a resonant driving type mirror to the resonant driving type mirror in the first section. The driving signal supply unit keeps the deflection angle of the resonant driving type mirror at the first target deflection angle in the second section. The driving signal supply unit supplies a driving signal of a frequency less than the resonance frequency of the resonant driving type mirror to the resonant driving type mirror in the third section. The optical scanning apparatus is used in an image forming apparatus.