A light beam scanning apparatus includes a sweeping unit for sweeping a light beam in response to a first signal. A photodetector has a plurality of optical gratings arranged in an array in a predetermined direction in which the light beam is swept. The photodetector generates a second signal in response to a movement of the light beam along the grating array. A control circuit generates the first signal in response to the second signal. The control circuit detects a scan speed of the light beam based on the second signal, and adjusts the first signal in response to the detected scan speed to control the scan speed of the light beam such that the detected scan speed substantially coincides with a predetermined speed.