In an image-forming apparatus having a photosensitive body and an intermediate transfer belt, the intermediate transfer belt is extendible/contractible in a longitudinal direction and a toner image formed on the photosensitive body transferred onto the intermediate transfer belt. The intermediate transfer belt has a mark formed from photo-reflective particles on one or both sides thereof, which mark has a different photo reflectivity to the intermediate transfer belt and is used for detecting a reference position of the intermediate transfer belt.