A rotary arm restricts the rotation of a first sun gear or a second sun gear according to forward/reverse rotation drive of sheet discharge rollers. As a result, a rotating direction of a transmission gear coupled to the sheet discharge rollers is switched. If 1A denotes a pressure angle of gear teeth of a gear portion of the first sun gear and 2A denotes a pressure angle of gear teeth of a first planetary gear, a relationship of 2A+0.51A2A+1.5 is satisfied.

A rotary arm restricts the rotation of a first sun gear or a second sun gear according to forward/reverse rotation drive of sheet discharge rollers. As a result, a rotating direction of a transmission gear coupled to the sheet discharge rollers is switched. If 1A denotes a pressure angle of gear teeth of a gear portion of the first sun gear and 2A denotes a pressure angle of gear teeth of a first planetary gear, a relationship of 2A+0.51A2A+1.5 is satisfied.