An actuator includes a piston having an actuating pin, a squib, and a cylinder case. The cylinder case has a peripheral wall part that covers the circumference of the piston and a ceiling wall part having a through hole which allows an actuating pin to protrude from the through hole. The actuator is configured to make the a piston move backward along with the actuating pin by ejecting a driving gas generated from the squib to a space between the ceiling wall part and the piston during actuation. A sealing ring which is relatively movable with respect to the actuating pin while maintaining the state of sealing up a gap between the actuating pin and the through hole, is provided at an inner face side of the ceiling wall part.