To provide an optical scanner in which the reference scanning position of a light beam is corrected even when the reference scanning position of the light beam in a subscanning direction with a deflection mirror having a large time constant as response characteristic varies, and to provide an image forming apparatus.

The optical scanner includes: a light beam generation part 44 having a matrix light emitting element structure which generates a plurality of light beams; a deflection mirror 40 which deflects and emits the incident light beams; a signal detection part 49 which detects the light beams; a light beam on/off control means which controls the light beam generation part 44 so that the light beams deflected with the deflection mirror 40 are turned on and off in a specific optical scanning region while reciprocally scanning, wherein an arbitrary optical element of the matrix light emitting element structure is selectable as the light source of the light beams according to the detection signal of the signal detection part 49, and the light beam on/off control means turns on and off the light source.