This invention provides an image forming apparatus capable of effectively suppressing an AC color registration error, and a control method thereof. To accomplish this, the image forming apparatus according to this invention controls driving of a transfer belt so that the speed of surface of the transfer belt at the transfer position comes close to a constant speed. The image forming apparatus also controls driving of a photosensitive drum so that the error between the speed of surface of the photosensitive drum and that of the transfer belt at the transfer position becomes almost 0. Further, the image forming apparatus controls the exposure timing in accordance with fluctuations of the speed of surface at the exposure position so that the exposure interval in the sub-scanning direction on the photosensitive drum comes close to a constant interval.