An image forming apparatus has a movable image carrier for carrying a toner image thereon; an image forming section for forming the toner image on the movable image carrier; a transfer section for electrostatically transferring the toner image on the movable image carrier to a transfer medium at a transfer position; an electrifying section for electrifying the toner image on the movable image carrier before the toner image reaches the transfer position; and a bias applying section for applying a bias to the electrifying section. This image forming apparatus controls the bias output of the bias applying section in accordance with information about surface roughness of the transfer medium.