Image forming apparatus having biased transfer roller

An image forming apparatus has a charger for charging a photosensitive body; a developing device for developing an electrostatic latent image formed on the photosensitive body to form a toner image; and a transfer device for transferring the toner image to a transferred material by a transfer roller to which a bias having a polarity reverse to that of toner is applied. At a pre-rotating time of the transfer roller, a bias having the same polarity as toner is applied to the transfer roller for a time equal to or greater than a time required to rotate the transfer roller at one time such that it is possible to prevent normal toner from being attached to the transfer roller. After a constant time has passed, a voltage of the transfer roller is set to zero volt so that reversed toner attached to the transfer roller is moved to the photosensitive body to which the bias having polarity reverse to that of toner is applied.