IMAGE FORMING APPARATUS HAVING A MOVABLY SUPPORTED TRANSFER BELT DETECTOR

The image forming apparatus includes an image bearing member bearing a toner image; a driving roller and a tension roller movable in such a direction that the interval between the rollers changes; a transfer belt stretched by the rollers to feed a receiving material so that the toner image on the image bearing member is transferred onto the receiving material or to directly receive the toner image from the image bearing member; a detector opposed to the tension roller with the belt therebetween to optically detect a mark on the belt, wherein the detector is movably supported while biased toward the tension roller, and the moving direction of the tension roller is substantially the same as an optical axis of light emitted by the detector; and a regulator to regulate movement of the tension roller and the detector in a direction perpendicular to the moving direction of the tension roller.