An actuator capable of realizing a high output with a compact size is proposed. An actuator provided with a motor including a cylindrical rotor, and a reducer including an input shaft coaxial with a rotational shaft of the motor and nested in the rotor. The reducer has a cylindrical shape, the reducer further includes an output shaft coaxial with the rotational shaft of the motor, the motor further includes a stator, and the actuator is further provided with a casing which supports the reducer and the stator.