A developing device configured such that the developing device or the like can be easily inserted and removed without increasing the size of the apparatus, and an image forming apparatus that uses this developing device as well as a process cartridge. The developing device is provided with a first rib formed projecting from a first main wall surface, which is the side surface with the maximum area in the chassis of the developing device, and a second rib formed projecting from a second main wall surface, which is the side surface in opposition to the first main wall surface, the first rib being disposed in a position higher than the second rib in the height direction.