An image forming apparatus which attains a high quality image by suppressing fluctuation in speed of an intermediate transfer belt generated when a leading edge of a recording material having high rigidity such as a thick sheet is brought into contact with the belt in a single color mode. The single color mode is switched between a first single color mode in which a toner image on one photosensitive drum is primarily transferred onto the belt with only the one drum being in contact with the belt, and the toner image on the belt is secondarily transferred onto the recording material, and a second single color mode in which a toner image born on one of drums is primarily transferred onto the belt with the drums being in contact with the belt, and the toner image on the belt is secondarily transferred onto the recording material.