An image forming apparatus is operable in a mode in which a speed of an intermediary transfer belt is changed after all of the toner images are primary-transferred onto the intermediary transfer belt, and the toner image primary-transferred on the intermediary transfer belt is passed through a secondary transfer position and primary transfer position in this order to reach the secondary transfer position, where the toner image is secondary transferred onto the transfer material, and a controller for making, before the image forming apparatus is operated in the mode to change the speed of the intermediary transfer belt, a potential difference between the intermediary transfer belt and the photosensitive member at the primary transfer position smaller than a potential difference between the intermediary transfer belt and the photosensitive member when the toner image is primary transferred from the photosensitive member onto the intermediary transfer belt.