A tandem type photosensitive unit detachably mounted in a main body of an image forming apparatus is provided. The tandem type photosensitive unit includes a plurality of parallelly arranged photosensitive drums; developer cartridges mounted on respective cartridge mounting positions having a constant positional relationship to photosensitive drums respectively; a pair of first side plates opposed to each other in an axial direction of the photosensitive drums for positioning each photosensitive drum by retaining both end portions of each photosensitive drum in the axial direction; and a pair of second side plates opposed to each other in the axial direction for retaining each developer cartridge mounted on the cartridge mounting position. The pair of second side plates are provided with guided portions guided to a unit guide portion provided in the main body and cartridge guide portions for guiding mounting of each developer cartridge on the cartridge mounting position.