An injector includes a syringe unit including a syringe that encapsulates an injection objective substance, an ignition device, and a piston, and an activating unit that activates the syringe unit. The syringe unit includes a cylindrical housing, in which the ignition device and the piston are incorporated, and can be installed in a housing space formed in the housing through an opening section provided in a side surface of the housing. The syringe includes a nozzle and a plunger that forms a sealed space for encapsulating the injection objective substance and that is capable of being propelled toward the nozzle by the piston. When the syringe is installed in the housing, the syringe is fixed to dispose the plunger of the syringe in a position opposed to the piston incorporated in the housing.