A plurality of image bearing members is supported by an apparatus main body and is arranged in a predetermined direction. A belt unit is configured to be detachably mounted on the apparatus main body. The belt unit includes a belt frame, a plurality of belt support rollers, a belt, and a plurality of transfer members. The plurality of belt support rollers is supported by the belt frame. The belt is movably supported by the plurality of belt support rollers and extends in the predetermined direction. Each of the plurality of transfer members is disposed in confrontation with a corresponding one of the plurality of image bearing members with the belt interposed therebetween. A positioning member fixes, relative to the apparatus main body, each of the plurality of transfer members at a position in the predetermined direction when the belt unit is mounted on the apparatus main body.