a plurality of light sources generate beams having predetermined light amounts , respectively . an optical scanner receives the beams from the plurality of light sources and directs the beams as a plurality of scanning beams to an object to be scanned . a photodetector has a predetermined number of photosensors smaller than that of the plurality of light sources and substantially detects light amounts of the plurality of scanning beams by the optical scanner in a time sharing manner . a controller outputs a control signal for independently controlling the light amounts of the respective beams from the plurality of light sources in accordance with time - sharing light amount detection signals corresponding to the plurality of scanning beams from the photosensors of the predetermined number .