NOVELTY - A fluid (F) containing fuel, water vapor and/or carbon-dioxide-containing gas, and oxygen-containing gas is supplied to reforming reaction region (I). The fuel is contacted with catalyst (C1) at reaction region temperature, whose upper limit is thermal decomposition index temperature of fuel defined with type and concentration of hydrocarbon in fuel, and a reformed gas containing methane, hydrogen and carbon monoxide is manufactured.    USE - For manufacturing reformed gas used for manufacturing liquid fuel and hydrogen used for fuel cell.    ADVANTAGE - The reforming gas is efficiently and reliably manufactured, and decrease of catalytic activity is prevented.    DETAILED DESCRIPTION - A fluid (F) containing fuel, water vapor and/or carbon-dioxide-containing gas, and oxygen-containing gas is supplied to reforming reaction region (I). The fuel contains 2C or more hydrocarbon. The fuel is contacted with a reforming catalyst at reaction region temperature, whose upper limit is thermal decomposition index temperature of fuel defined with type and concentration of hydrocarbon contained in fuel, and reformed gas containing methane, hydrogen and carbon monoxide is manufactured. An INDEPENDENT CLAIM is included for reformed-gas manufacturing apparatus. The apparatus has a reaction chamber for accommodating reforming catalyst, a supply path for supplying the fluid (F) to the reaction chamber, a reformed-gas outlet for deriving reformed gas produced by reforming reaction from reaction chamber, a reaction room temperature control unit (A) for controlling the temperature of reaction chamber. The unit (A) carries out cooling of reaction chamber, when the maximum temperature of reforming reaction region exceeds upper limit temperature.    DESCRIPTION OF DRAWING(S) - The graph shows the temperature distribution of modifier of reformed gas manufacturing apparatus. (Drawing includes non-English language text).    low-temperature reformer (1)    high-temperature reformer (2)