An indicator includes a case mountable in a gas filling passage of a pressure container; a valve element which is urged toward one side of the case connectable to the pressure container, the valve element being movable in the case to a position balanced with gas pressure in the pressure container; an O-ring serving as a seal member for ensuring gas tightness between the valve element and the case; and an indicating member slidable relative to the case between a position protruding outside the case and a position not protruding. The O-ring is arranged on the valve element so that the O-ring is in contact with the case to ensure gas tightness when the valve element is in a first balanced position while a gas pressure in the pressure container is e.g. 7 MPa or higher and the O-ring is out of contact with the case not to ensure gas tightness when the valve element is in a second balanced position while the gas pressure is less than e.g. 7 MPa.