An injector can include an ignition device including a partition wall member forming a first space for housing a gunpowder and made from a rigid material to be destroyed by a rise in pressure in the first space in a case that the gunpowder is combusted. The actuator can also include a casing including a base portion being fixed to the actuator body near the ignition device, and being disposed in a space inside the actuator body covering the ignition device. The casing can define a second space between the casing and the partition wall member of the ignition device, and seal, inside the second space, a combustion product generated by combustion of the gunpowder by the ignition device. When pressure inside the second space arises due to combustion of the gunpowder, a portion of the casing can stretch to approach a predetermined end portion of the output piston portion.