The authors develop a small and simple steam-reforming reactor in a home-use size for n-dodecane as a heavy-hydrocarbons fuel. Under the thermal condition controlled by electric heaters and a gas burner with a thermal diffuser, the authors measure the inside-temperature profile and the hydrogen molar fraction (concentration) C H2 , together with the molar fractions C CH4 , C CO and C CO2 of other main gas components such as CH 4 , CO and CO 2 , respectively. As a result, the authors successfully achieve suitable inside-temperature profiles. And, the authors experimentally reveal the effects of the liquid-hourly space velocity LHSV upon the molar fractions, in addition to the LHSV effect upon a conversion ratio X C12H26 . Furthermore, the authors reveal the effects of the temperature T inside the reactor upon both the molar fractions and X C12H26 , comparing with the chemical-equilibrium theory in a wide range of LHSV. The comparison with the chemical-equilibrium theory indicates that the temperature at the catalyst-layer-bed downstream end could be regarded as the reaction temperature.