NOVELTY - The gas physical quantity detecting device for a fuel cell system of a vehicle, comprises an element for detecting the physical quantity of the gas flowing through a gas flow passage, a detection element chamber (7) containing the detection element for supplying gas from inside the gas flow passage to the detection element, and a filter (6) arranged between the gas flow passage and the chamber. The filter has a predetermined thickness. The filter is made of a non-hydrophobic material having fibers interwoven in a non-uniform manner such that the filter has a three dimensional form.    USE - The gas physical quantity detecting device is useful for a fuel cell system of a vehicle (all claimed).    ADVANTAGE - The device is capable of effectively and accurately detecting the gas physical quantity without the occurrence of poor detection results caused by water condensation.    DETAILED DESCRIPTION - The gas physical quantity detecting device for a fuel cell system of a vehicle, comprises an element for detecting the physical quantity of the gas flowing through a gas flow passage, a detection element chamber (7) containing the detection element for supplying gas from inside the gas flow passage to the detection element, and a filter (6) arranged between the gas flow passage and the chamber. The filter has a predetermined thickness. The filter is made of a non-hydrophobic material having fibers interwoven in a non-uniform manner such that the filter has a three dimensional form. The fibers are arranged to form gaps between the fibers on a surface of the filter that faces the chamber in which the gaps are smaller than a maximum flame extinguishing diameter of a hydrogen flame. The filter has a multiple layered structure with a space provided between adjacent layers, a fiber density on a side of the filter nearer to the chamber that is lower than a fiber density on a side of the filter nearer to the gas passage, a part protruding into the gas flow passage, and a recessed shaped surface that faces the chamber. The detection element chamber has an internal wall surface with a water repellent treatment or a hydrophilic treatment. The internal wall surface of the chamber has a surface roughness that is larger than a surface roughness of a surface of the cover member. The filter extends along the internal side wall surface of the chamber. The detection element includes a cover member with a surface of the cover member facing the chamber, and a first sensor in a first cover and a second sensor in a second cover. The first cover has a gas introducing hole. The second sensor is a reference sensor, and contains a reference gas. The first and second cover members are spaced apart with a gap between the cover members inwardly from the internal side wall of the chamber. INDEPENDENT CLAIMS are included for:    (1) a fuel cell system; and    (2) a vehicle.    DESCRIPTION OF DRAWING(S) - The diagram shows a partial cross sectional view of a gas physical quantity detecting device.    Gas pipe (2)    Sensor mounting part (4)    Filter (6)    Detection element chamber (7)    Element cover. (9)