NOVELTY - A proton conductor comprises a porous sintered body made of tetravalent metallic oxide, where pyrophosphate as a tetravalent metallic compound is further formed on surfaces of the porous sintered body, porous walls of the porous sintered body, and in the inside of each pore of the porous sintered body.    USE - As proton conductor (claimed), for an electrolyte used in a fuel cell.    ADVANTAGE - The proton conductor has a superior air-tight capability, and prevents gas leakage in the carbon quantity detecting sensor from occurring. This provides and guarantees long life and durability of the carbon quantity detecting sensor with high reliability for a long period of time. The proton conductor is made of the porous sintered body has the capability of superior mechanical strength and the capability of air-tightness when compared with a conventional proton conductor made of a press-formed mold body.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) producing a proton conductor involving contacting a porous sintered body made of tetravalent metallic oxide with liquid solvent containing phosphate, and heating the porous sintered body so that the pyrophosphate as a tetravalent metallic compound is formed on a surface and porous wall and in an inside of the pores of the porous sintered body; and    (2) carbon quantity detecting sensor placed in a gas flow passage through which a measuring gas containing carbon component flows, and detecting a quantity of carbon contained in the measuring gas, comprising the proton conductor, a pair of a measuring electrode and a reference electrode formed on both surfaces of the proton conductor so that the measuring electrode faces the measuring gas and the reference electrode is apart from the measuring gas; and an electric power source configured to supply a predetermined current or a predetermined voltage to the electrode pair composed of the measuring electrode and the reference electrode, where each of the measuring electrode and the reference electrode is comprised of one of a porous metallic electrode and a cermet electrode, the porous metallic electrode containing one of gold (Au), platinum (Pt), palladium (Pd) and silicon carbide (SiC).    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of scanning electron microscope photograph of proton conductor.