A power unit comprises: a wave gear speed reducer, a rotary shaft extending in the axial direction; a rotor unit that rotates integrally with the rotary shaft; a stator unit disposed facing the rotor unit; and a motor casing to which the stator unit is secured. The motor casing has a second cover that covers the rotor unit and the stator unit from the other side in the axial direction. The rotary shaft extends toward the other side in the axial direction, penetrates the second cover, and is connected to a cam. The second cover has a support part that extends toward the other side in the axial direction, supports the rotary shaft in a rotatable manner, and is positioned in a flexible cylindrical part of an external gear.