NOVELTY - Hydrogen gas and/or lower hydrocarbon gas concentration measuring apparatus (10) has catalyst unit (50) having catalyst accommodating portion (52), and heating unit (54). Unit (54) heats catalyst of catalyst receptacle interior portion to preset temperature. To-be-measured gas is supplied to heated catalyst, and catalyst is reacted with hydrogen gas and/or lower hydrocarbon gas in mixed gas. The concentration is measured.    USE - For measuring lower hydrocarbon gas and/or hydrogen gas (claimed) used in hydrogen storage and supply plant, fuel cell system and hydrogen station for motor vehicles.    ADVANTAGE - The hydrogen gas and/or lower hydrocarbon gas measuring device is compact, and has explosion-proof structure.    DETAILED DESCRIPTION - Hydrogen gas and/or lower hydrocarbon gas concentration measuring apparatus (10) has unit having light interferometer unit (20) for dividing light ray from light source (22) into two light rays, standard-gas chamber (28), unit for passing to-be-measured mixed gas through chamber, unit for accumulating light rays, and unit for obtaining gas concentration by measuring interference fringe produced during accumulation of light ray. The apparatus has a catalyst unit (50) having catalyst accommodating portion (52), and a heating unit (54). The heating unit heats catalyst of catalyst receptacle interior portion to predetermined temperature. The to-be-measured gas is supplied to catalyst heated at predetermined temperature, and catalyst is reacted with hydrogen gas and/or lower hydrocarbon gas in mixed gas. The concentration of hydrogen gas and/or lower hydrocarbon gas in mixed gas is measured. An INDEPENDENT CLAIM is included for method for measuring concentration of hydrogen gas and/or lower hydrocarbon gas.    DESCRIPTION OF DRAWING(S) - The figure shows the outline of device for measuring hydrogen gas and/or lower hydrocarbon gas. (Drawing includes non-English language text).    hydrogen gas and/or lower hydrocarbon gas concentration measuring device (10)    light interferometer unit (20)    light source (22)    standard-gas chamber (38)    to-be-measured mixed gas chamber (40)    catalyst unit (50)    catalyst accommodating portion (52)    heating unit (54)