A photosensitive unit is provided. The photosensitive unit includes a frame, plural photosensitive drums which are held by the frame, plural cartridges detachably mounted in the frame. Each cartridge includes a developing roller for supplying developer to the corresponding photosensitive drum, a supply roller, and a casing having a first casing portion supporting the developing roller and the supply roller and including a first wall, and a second casing portion storing developer and including a second wall connected to the first wall through a first stepped portion. The frame includes plural beam members which are provided between a pair of side parts. Each beam member includes a third wall which opposes the first wall with a distance therebetween, and a fourth wall which opposes the second wall with a distance therebetween in a state where a cartridge is mounted in the frame.