In a black monochromatic mode, an intermediary transfer belt is separated from an upstream photosensitive drum in a state in which a downstream photosensitive drum contacts the intermediary transfer belt. At this time, a stretching roller is moved upwardly in interrelation with a contact-separation mechanism to increase an angle of contact of the intermediary transfer belt with the downstream photosensitive drum compared with the case of a full-color mode. On the other hand, in the full-color mode, the stretching roller is moved downwardly to be separated from an inner surface of the intermediary transfer belt.