An image forming apparatus includes gears for driving a first photosensitive drum and a second photosensitive drum. The gears includes a first drum gear, a second drum gear, and a first drive gear configured to apply a rotational force to the first drum gear and the second drum gear by meshing with the first drum gear and the second drum gear. Specific positional relations among the first photosensitive drum, the second photosensitive drum, the first drum gear, the second drum gear, and the first drive gear are configured based on exposure positions and transfer position. Such an image forming apparatus may reduce an occurrence of an image formation failure even when the dimensional unevenness of the gears occurs.