The image forming apparatus, comprising:
- a plurality of N image carriers, including first through Nth image carriers, to carry toner images formed thereon, satisfying a relation of N ≧2;
- a transfer unit including an endless transfer belt an surface of which contacts the plurality of N image carriers, to successively transfer the toner images formed on the plurality of N image carriers onto either the outer surface of the endless transfer belt or a surface of a recording medium carried on the endless transfer belt;
- a plurality of M belt supporting members disposed in contact with an inner surface of the endless transfer belt to support the endless transfer belt, satisfying a relation of M >N,
- the 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 nip opposing member defining a first supported area of the endless transfer belt in a circumferential direction thereof where the first nip opposing member contacts and supports the endless transfer belt;
- a contact and separation mechanism including a retaining unit that supports at least the first nip opposing member,
- contact and separation mechanism moving the 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 provided in proximity to the endless transfer, fixedly mounted on the retaining unit, and moved by the contact and separation mechanism together with the retaining unit that supports at least the first nip opposing member,
- the mark detector detecting a plurality of adjacent marks formed at a predetermined pitch on the endless transfer belt in the circumferential direction of the endless transfer belt,
- wherein the retaining unit includes a first sub-retainer that retains the first nip opposing member and a second sub-retainer that retains the plurality of N nip opposing members other than the first nip opposing member,
- an extreme upstream belt supporting member being disposed adjacent to and upstream from the Nth nip opposing member in the direction of rotation of the transfer belt and fixedly mounted on the second sub-retainer, the Nth nip opposing member and the Nth image carrier together forming and extreme upstream transfer nip with the endless transfer belt interposed therebetween,
- an extreme downstream belt supporting member being disposed adjacent to and downstream from the first nip opposing member in the direction of rotation of the endless transfer belt and fixedly mounted on the first sub-retainer,
- the endless transfer belt being tensioned in a straight line by and between the extreme upstream belt supporting member and the extreme downstream belt supporting member in the direction of rotation of the endless transfer belt,
- a second upstream belt supporting member being disposed adjacent to and upstream from the first nip opposing member in the direction of rotation of the endless transfer belt and fixedly mounted on the first sub-retainer,
- the mark detector being fixedly mounted on the first sub-retainer between the first nip opposing member and the second upstream belt supporting member to detect the plurality of marks formed on the endless transfer belt while the plurality of marks pass through a mark detection range defined by the extreme downstream belt supporting member and the second upstream belt supporting member,
- the mark detection range extending from an area supported by the second upstream belt supporting member to an area supported by the extreme downstream belt supporting member in the direction of rotation of the endless transfer belt.