An actuator includes a hollow cylinder which has a first portion and a second portion. The inner diameter of the first portion is larger than that of the second portion. A piston is inserted in the cylinder. The actuator has a gas generator which supplies a gas into the cylinder to move the piston toward the first portion. The seal member keeps airtightness between the piston and the cylinder when the piston stays in the second portion and flows the gas when the piston stays in the first portion. A hollow stopper is arranged in the first portion and stops the piston. The stopper has a vent groove which is provided on the inner surface and connects the interior of the cylinder to the open end. A piston rod is jointed to the piston and housed in the cylinder. The piston rod moves along the axis with the piston.