In a registration step, the temperature near a polygon mirror (83) is detected by a first temperature sensor (85), and the internal ambient temperature of an image forming apparatus (1) is detected by a second temperature sensor (86). Also, a registration adjustment value of each color is determined and stored in a memory (75). In a printing step, the registration adjustment value (A) of each color determined in the registration step is corrected based on a change in the temperature near the polygon mirror (83) and a change in the internal ambient temperature of the image forming apparatus (1). Furthermore, the write timing of an electrostatic latent image onto each photosensitive drum (13) by an optical scanning device (11) is controlled. Then, the position of the electrostatic latent image on each photosensitive drum (13) is shifted by an amount corresponding to the corrected registration adjustment value.