The present invention relates to a hydrocarbon-decomposing catalyst comprising magnesium, aluminum and nickel as constitutional elements, containing silicon in an amount of 0.001 to 20% by weight (calculated as Si) based on the weight of the catalyst, and having a nickel content of 0.1 to 40% by weight (calculated as metallic nickel) based on the weight of the catalyst; and a hydrocarbon-decomposing catalyst comprising a porous carrier and a catalytically active metal carried on the carrier, said porous carrier comprising magnesium and aluminum as constitutional elements and containing silicon in an amount of 0.001 to 20% by weight (calculated as Si) based on the weight of the catalyst, and said catalytically active metal comprising fine metallic nickel particles, a content of said fine metallic nickel particles being 0.1 to 40% by weight (calculated as metallic nickel) based on the weight of the catalyst. The hydrocarbon-decomposing catalyst of the present invention is less expensive, and can exhibit an excellent catalytic activity capable of decomposing C1 or more hydrocarbons, a good anti-coking property even under a less steam condition, a good effect of reducing an amount of ammonia by-produced even upon using nitrogen-containing hydrocarbon fuels, and a sufficient strength capable of preventing the catalyst from being crushed and broken even upon occurrence of coking inside thereof, and an excellent durability.