NOVELTY - A hydrocarbon-decomposing catalyst (C1) comprises magnesium, aluminum, nickel and silicon, where the magnesium and aluminum constitutes a part of a porous carrier. The silicon is contained in the carrier in an amount of 0.001 - 20 wt.% calculated as silicon based on the weight of the catalyst. The nickel in the form of fine metallic nickel particles (0.1 - 40 wt.% calculated as metallic nickel based on the total weight of the catalyst) is carried as catalytically active metal on the porous carrier.    USE - As hydrocarbon-decomposing catalyst for decomposing hydrocarbons, and in a power generation system for production of hydrogen (claimed), which is a fuel for fuel cells and for internal combustion engines of automobiles. The power generation system includes phosphoric acid type (PAFC), solid polymer type (PEFC), solid oxide type (SOFC) and fuel cell systems for micro-gas turbine and automobiles.    ADVANTAGE - The hydrocarbon-decomposing catalyst is less expensive; exhibits an excellent catalytic activity capable of decomposing hydrocarbons such as 1C hydrocarbon, at least 2C hydrocarbons; exhibits a good anti-coking property even under a less steam condition; has sufficient strength to prevent crushing or breaking even upon occurrence of coking inside it; and has excellent durability. The catalyst prepared by supporting specific amount of fine metallic nickel particles on porous carrier composed of specific composite oxide containing specific amount of silicon, exhibits catalytic activity capable of decomposing hydrocarbons. The catalyst is capable of reducing amount of ammonia by-product by the reaction between nitrogen compounds contained in trace amount in hydrocarbon raw material and hydrogen produced after the hydrocarbon decomposition reaction. The catalyst can effectively decompose hydrocarbons. The catalyst exhibits specific surface area of 7 - 320 (preferably 20 - 280) m2/g; has sufficient catalytic activity, durability, anti-coking property and anti-sulfur-poisoning property even when an autothermal reforming reaction is used; has enhanced sintering property.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) decomposing (M1) hydrocarbons by adding steam to hydrocarbon-containing gas, involving: contacting the hydrocarbon-containing gas and the steam with the hydrocarbon-decomposing catalyst at 250 - 850degreesC under such condition in which molar ratio of steam to carbon is 1 - 6;    (2) producing (P1) hydrogen involving: decomposing hydrocarbons by the decomposing method (M1);    (3) a power generation system in which hydrogen obtained by process (P1) is electrochemically reacted with oxygen; and    (4) decomposing hydrocarbons by adding steam to gas containing desulfurized at least 1C hydrocarbon, involving: a low-temperature decomposition step of decomposing mainly at least 2C hydrocarbon contained in the hydrocarbon containing gas to obtain a gas containing 1C hydrocarbon as main component; and a high-temperature decomposition step of decomposing the 1C hydrocarbon contained as main component in the hydrocarbon-containing gas obtained in the low-temperature decomposition step, where in the low-temperature decomposition step and/or the high-temperature decomposition step, the hydrocarbon-containing gas and the steam are contacted with the hydrocarbon-decomposing catalyst.