An image forming apparatus includes a plurality of photosensitive drums, a plurality of developing devices, an intermediate transfer medium, primary transfer members, and a secondary transfer member for transferring toner images onto a transfer material. A full-color mode and a monocolor mode are selectively carried out. The image forming apparatus also includes a CPU for adjusting the length of a non-image forming region, and an intermediate-transfer-medium movement controller. The CPU brings about a switching state in which an image forming region lies between a primary transfer position on the most downstream one of the photosensitive drums and a secondary transfer position and in which non-image forming regions simultaneously lie at the primary transfer position and at the secondary transfer position. The intermediate-transfer-medium movement controller switches between the full-color mode and the monocolor mode during the switching state.