A driving force transmission unit includes a first coupling, a second coupling, and a first rotary shaft. The first coupling includes a first contact portion and a hole, the inner circumferential surface of which has a rounded rectangular shape in cross section including a circular arc portion. The second coupling includes a second contact portion disposed at a position facing the first contact portion to contact the first contact portion along a direction of rotation of the second coupling. The first rotary shaft is connected to the first coupling and includes a shaft portion, an outer circumferential surface which has a rounded rectangular shape in cross section including a circular arc portion inserted into the hole of the first coupling.