The drive of direct-heat-supply type reforming of hydrocarbon at ordinary temperature is necessary in order to realize a self-sustaining, on-site reforming type fuel cell system which does not necessitate the supply of energy from the outside. According to the invention, an oxide, CeO2 or Pr6O11, or a Ce/Zr or Ce/Zr/Y double oxide is used as the oxide containing a rare earth element capable of changing the oxidation number with an active metal and oxygen defects are introduced into the oxide or double oxide by activating the oxide or double oxide with a reducing gas at high temperature. When a reaction gas containing hydrocarbon and oxygen is passed though the catalyst at low temperature, the oxygen defects react with oxygen and thereby return to the original oxide. Since this return reaction is an exothermic reaction, the catalyst itself is heated, which acts as the driving force for advancing the combustion of the hydrocarbon, whereby the catalyst layer is further heated and the reforming is advanced to from hydrogen. Thus, the hydrogen-producing reaction can be driven even at low temperature, particularly ordinary temperature.