An image forming apparatus including a main body casing, a drum cartridge and a developing cartridge. The main body casing has an opening and a slot extending in from the opening toward an inside of the main body casing along a first direction. The drum cartridge and the developing cartridge are arranged in a second direction intersecting the first direction in a state where the drum cartridge and the developing cartridge are attached to the slot. The main body casing includes a drum guide groove configured to guide movement of the drum cartridge. The drum cartridge includes a drum protrusion configured to be engaged with the drum guide groove. A width of the drum guide groove in the second direction is decreased toward the inside of the main body casing from the opening.