An image forming apparatus has an image carrier carrying a toner image; a transfer member made with an ion conductive material and forming a transfer nip by being pressed by the image carrier; and a power supply continuously applying a transfer bias voltage to the transfer member as a plurality of print media pass through the transfer nip. The transfer bias voltage has a predetermined polarity. A control section determines whether the resistance of a nip-margin area has exceeded a predetermined resistance threshold, the nip-margin area being a marginal portion of the transfer nip through which no print medium passes. When the determination of the control section is affirmative, the power supply applies a reverse-bias voltage to the transfer member, the reverse-bias voltage having an opposite polarity to the transfer bias voltage.