A transmission which includes a housing, a planetary gear mechanism, an input shaft, an output shaft, a brake including a friction member, a piston member pressing the friction member, and an actuator portion applying a pressing force to the piston member, the brake braking a rotation of one of a sun gear, a ring gear, and a carrier of the planetary gear mechanism, an external output gear positioned between the friction member and the actuator portion in a direction of a rotation axis, the external output gear provided at a member integrally rotating with the input shaft, and the piston member including an opening portion formed over a predetermined angular range about the rotation axis and within a range in the direction of the rotation axis in which the external output gear is disposed.