An object of the present invention is to prevent deterioration of image quality caused by ghost light generated from components of a scanning optical apparatus. In the present invention, an image writing light flux is related with a position at which ghost light appears and the exposure amount of the ghost light and a ghost light exposure amount on a surface to be scanned is calculated on comparison with an actual image signal, and the light quantity or the pulse width of a light flux emitted from a light source is controlled based on the calculated ghost light exposure amount.