Abstract: An ion thruster is operable in an interplanetary space with plasma generated by microwaves in a propellant atmosphere. A vessel defines first, second and third hollow spaces and a window between the first hollow space and the second and third hollow spaces, the second hollow space having an opening. A microwave generating unit generates the microwave in the first hollow space as a standing wave penetrating into the second and thid spacers hollow. A propellant supplying unit supplies the propellant into the second and third hollow spaces, the propellant serving as a main and neutralizing propellant and absorbing the standing wave to produce main plasma comprising main ions and main elecrons. An accelerating unit accelerates only the main ions into an ion beam to inject the ion beam through the opening into the interplanetary space. A neutralizing unit defines a third space which is in communication to the first space and into which the standing wave penetrates.