An image forming apparatus includes an image bearer, a transfer device, and a transfer bias power source. The transfer bias power source is configured to output an alternately switching voltage that alternates between a transfer-directional voltage having a polarity to transfer the toner image from the image bearer onto the recording medium and a return-directional voltage having an opposite polarity to the polarity of the transfer-directional voltage. The transfer bias power source is configured to reduce a target value of Vdc as an image area ratio of a toner image to be printed increases, and to increase the target value of Vdc as the image area ratio reduces, where Vdc is an average value of the alternately switching voltage during printing.