An image forming apparatus capable of forming images on both sides of a recording medium of the present invention includes a first image carrier on which a toner image to be formed, and a second image carrier to which the toner image is transferred from the first image carrier. The toner image transferred from the image carrier to the second image carrier is transferred to one side of the recording medium while a toner image is transferred from the first image carrier to the other side of the recording medium. After the toner image has been transferred from the first image carrier to the second image carrier, the running condition of the second image carrier is varied.