An actuator having an actuator body with a through-hole formed in an axial direction and an output piston portion slidably disposed inside the through-hole, the output piston portion being configured to protrude from an output surface of the actuator body and apply a predetermined force to a target object, wherein the actuator further comprises, an ignition device comprising a partition wall member forming a first space for housing a gunpowder and made from a predetermined rigid material so as to be destroyed by a rise in pressure in the first space in a case that the gunpowder is combusted; and a casing, wherein the base portion of the casing being fixed to the actuator body near the ignition device, and being disposed in a space inside the actuator body covering the ignition device, defining a second space between the casing and the partition wall member of the ignition device, and sealing, inside the second space, a combustion product generated by combustion of the gunpowder by the ignition device; wherein the casing comprises a pressing portion and a stretchable portion, wherein: the stretchable portion is configured to, via a rise in pressure inside the second space from combustion of the gunpowder inside the ignition device, stretch in an approaching direction so as to cause the pressing portion of the casing to approach a predetermined end portion of the output piston portion opposite an end portion that protrudes from the output surface, and the pressing portion is configured to press the predetermined end portion of the output piston portion via stretching of the stretchable portion; characterized in that the second space houses a gas generating agent for generating a predetermined gas by combustion; and the stretchable portion is configured to stretch in the approaching direction by a pressure rise in the second space from combustion of the gunpowder in the ignition device and combustion of the gas generating agent. wherein the actuator further comprises, an ignition device comprising a partition wall member forming a first space for housing a gunpowder and made from a predetermined rigid material so as to be destroyed by a rise in pressure in the first space in a case that the gunpowder is combusted; and a casing, wherein the base portion of the casing being fixed to the actuator body near the ignition device, and being disposed in a space inside the actuator body covering the ignition device, defining a second space between the casing and the partition wall member of the ignition device, and sealing, inside the second space, a combustion product generated by combustion of the gunpowder by the ignition device; wherein the casing comprises a pressing portion and a stretchable portion, wherein: the stretchable portion is configured to, via a rise in pressure inside the second space from combustion of the gunpowder inside the ignition device, stretch in an approaching direction so as to cause the pressing portion of the casing to approach a predetermined end portion of the output piston portion opposite an end portion that protrudes from the output surface, and the pressing portion is configured to press the predetermined end portion of the output piston portion via stretching of the stretchable portion; characterized in that the second space houses a gas generating agent for generating a predetermined gas by combustion; and the stretchable portion is configured to stretch in the approaching direction by a pressure rise in the second space from combustion of the gunpowder in the ignition device and combustion of the gas generating agent. the stretchable portion is configured to, via a rise in pressure inside the second space from combustion of the gunpowder inside the ignition device, stretch in an approaching direction so as to cause the pressing portion of the casing to approach a predetermined end portion of the output piston portion opposite an end portion that protrudes from the output surface, and the pressing portion is configured to press the predetermined end portion of the output piston portion via stretching of the stretchable portion; characterized in that the second space houses a gas generating agent for generating a predetermined gas by combustion; and the stretchable portion is configured to stretch in the approaching direction by a pressure rise in the second space from combustion of the gunpowder in the ignition device and combustion of the gas generating agent.