An image forming apparatus includes a belt unit having a driving roller, a driven roller, a driving roller gear; and a belt which is looped around the driving roller and the driven roller; a belt driving gear which is engaged with the driving roller gear; a photosensitive drum which contacts the belt; and a positioning section which contacts a portion of the belt unit to position the belt unit with respect to a moving direction of the belt. The positioning section contacts the portion of the belt unit in a direction of receiving a force with which the photosensitive drum is pressing the belt unit. The belt driving gear is engaged with the driving roller gear such that a force pressing the belt unit from the belt driving gear via the driving roller gear presses the belt unit against the positioning section.