a piston configured to move in a driving direction owing to a pressure of a gas;

a driver configured to drive a driving member by moving integrally with the piston in the driving direction;

a wheel configured to move the driver in a direction opposite to the driving direction;

an electric motor configured to rotate the wheel due to rotation of the electric motor, thereby moving the driver in the direction opposite the driving direction;

a planetary gear device configured to reduce a rotation output of the electric motor, thereby outputting a reduced rotation output to the wheel, the planetary gear device including a gear train with a plurality of gear train stages arranged in series;

a gear housing configured to house the planetary gear device; and

an elastic member disposed between an internal gear of a last stage of the gear train stages of the planetary gear device and the gear housing, wherein:

the elastic member is configured to receive a force from the internal gear, the force being generated when the electric motor rotates;

the gear housing includes a first housing made of a resin and a second housing made of a metal, the first housing and the second housing being connected to each other in a direction parallel to a center axis of the internal gear; and

the second housing houses the internal gear of the last stage of the gear train stages.