An image forming apparatus includes an endless intermediate transfer belt rotatably stretched over a plurality of rollers, and a detector to read a pattern on the intermediate transfer belt. One of the rollers includes a first portion disposed opposite an area on the intermediate transfer belt including a pattern passing position, and a second portion not including a pattern passing position. The first portion has a diameter smaller than that of the second portion thereof, and an elastic material is superimposed on the second portion only. The detector is disposed between a second roller disposed upstream of the first roller and a third roller disposed downstream of the first roller, and opposite the intermediate transfer belt.