Method and apparatus for image forming capable of accurately detecting displacement of transfer images and image density

An image forming apparatus includes a plurality of image carriers, an image forming mechanism, a seamless belt-like intermediate transfer body or a transfer material carrier, a detection mechanism, and an optical detecting apparatus. The optical detecting apparatus includes at least a printed board, and a plurality of photodetectors formed of a light-emitting element, a light-receiving element and light-shielding members, which are mounted on the printed board. The optical detecting apparatus is disposed such that it detects a belt surface adjacent to rollers of the intermediate transfer body or the transfer material carrier, and either displacement of a transfer image or image density is detected by a plurality of photodetectors detecting pattern images which are transferred from the plurality of image carriers to the image transfer body or the transfer material carrier.