NOVELTY - A gas sensor on a wall of a gas passage comprises: a gas detector detecting a target gas contained in the gas flowing in the gas passage; a detector housing, which houses the gas detector and has a gas chamber into which the gas is introduced; and a cutoff unit controlling the introducing of gas into the gas chamber from the gas passage, and including a valve for opening and closing a through hole between the gas passage and the detector housing, a spring component, a housing for spring component and an end of valve, and a compressed air supply unit.    USE - The gas sensor is for locating on a wall of a gas passage, which is an off-gas line of a fuel cell. It is useful for a gas sensor system, and used for detecting the target gas, i.e. hydrogen, contained in the gas flowing in the gas passage, during the power generation of fuel cell (claimed). It is useful for a humid oil-gas e.g. air oil-gas and dilution gas of fuel cell, and for installation on a fuel cell system (S) useful for fuel cell-powered vehicle.    ADVANTAGE - The introduction of moister into the gas chamber can be prevented, by cutting off the communication between the gas passage and the gas chamber during the stop of the power generation of the fuel cell by the cutoff unit. Thus, since the adhesion of an unfavorable component, e.g. moisture and dew condensation water, to the gas detector can be prevented, the gas sensor becomes resistant to a trouble. An exposure time of the gas detector to moisture can be minimized, and thus a life time of the gas detector (gas sensor) can be prolonged.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for the following:    (1) a gas sensor system comprising a gas sensor, and a controller controlling the motion of the cutoff unit, where the controller controls the cutoff valve to introduce the gas into the gas chamber from the gas passage only when measuring the target gas; and    (2) controlling a gas sensor system including a gas detector, a gas chamber, a cutoff valve, and a controller, comprising controlling, by the controller, the cutoff valve to introduce the gas into the gas camber from the a passage; and detecting, by the gas detector, a target gas contained in the gas introduced into the gas chamber, where controller introduces the gas into the gas chamber from the gas passage only when measuring the target gas by controlling the cutoff valve.    DESCRIPTION OF DRAWING(S) - The figure is a schematic view showing the fuel cell system including a hydrogen sensor.    Anode side inlet line (3)    Anode side outlet line (5)    Circulation pipeline (22)    Hydrogen exhaust line (25)    Fuel cell system (S)