A reduction gear unit incorporatable in an image forming apparatus includes planetary gear mechanisms each including a sun gear, an outer gear, planetary gears arranged between the outer gear and the sun gear at equal intervals and meshing with the sun gear and the outer gear, and a carrier rotatable coaxially with the sun gear and the outer gear to rotatably support the planetary gears. An extreme upstream mechanism in the drive transmission direction includes an input element connected to a drive source. An extreme downstream mechanism includes an output element connected to a member outputting the drive force to a rotary member. Any mechanism other than the extreme downstream mechanism includes an output element connected to an input element of an adjacent mechanism located downstream of any other mechanism. The sun gear of the extreme downstream mechanism has a larger pitch diameter than any other mechanisms.