An image forming apparatus of the present invention includes a plurality of image carriers (2Y,2M,2C,2BK) arranged along a first intermediate image transfer belt (3). Toner images of different colors formed on the image carriers are sequentially transferred to the first intermediate image transfer belt (3) one above the other to complete a first image. The first image is transferred to a second image transfer belt (15) and then transferred to one side of a sheet. At the same time, a second image completed on the first image transfer belt (3) is directly transferred to the other side of the same sheet. First and second control means are respectively disposed in the loop of the first intermediate image transfer body and the loop of the second intermediate image transfer body. The apparatus of the present invention can form color images on both sides of the recording medium rapidly in accurate register.