An image forming apparatus includes a plurality of N image carriers including first through Nth image carriers, a transfer unit including an endless transfer belt, a plurality of M belt supporting members including a plurality of first through Nth nip opposing members contacting the inner surface of the endless transfer belt at positions corresponding to where the plurality of N image carriers contacts the outer surface of the endless transfer belt to form N transfer nips thereat, the first opposing member defining a first supported area of the endless transfer belt, a contact and separation mechanism including a retaining unit to move the first supported area of the endless transfer belt into and out of contact with the first image carrier, and a mark detector to detect a plurality of adjacent marks formed at a predetermined pitch in a circumferential direction of the endless transfer belt.