An optical scanning apparatus in which an exposing signal is generated from pattern data indicating a position to be exposed by a light beam on a surface-to-be-scanned. A corrected exposing signal is generated by varying the width of pulses of the exposing signal in accordance with a distance between an exposing area corresponding to the pulse and an optical axis of the scanning optical system. The light beam is emitted and modulated by the corrected exposing signal. The pulse width and/or the light quantity of the light beam is/are varied in accordance with the distance between the exposing area and the optical axis of the scanning optical system.