A gas processing apparatus includes a process chamber for installing a process target therein, a through-hole being air-tightly in communication with a gas supply pipe, an injector for supplying gas into the process chamber, a sleeve being engaged to an outer peripheral surface of the injector inside the through-hole, an annular sealing member engaged to the outer peripheral surface of the injector, an engagement surface facing the sealing member, and a pressing part for pressing the sleeve toward the outer side of the process chamber. The pressing part compresses the sealing member by exerting pressure to the engagement surface from an end surface of the sleeve toward the sealing member, so that an inside of the injector and an outside of the injector are air-tightly sealed.