There is conventionally known an image forming apparatus provided with a main casing, an intermediate transfer belt, a plurality of process units disposed below the intermediate transfer belt, and a drawer supporting the plurality of process units and drawable from the main casing. In this configuration, the process units each integrally include a photosensitive drum and a developing unit having a developing roller for supplying toner to the photosensitive drum and a toner chamber, and can each be replaced with new one by drawing the drawer from the main casing.