A catalyst for reacting hydrocarbon with steam according to the present invention, has a specific surface area of 40 to 300 m2/g, and comprises: a porous composite oxide carrier containing magnesium and aluminum; and nickel metal in the form of fine particles carried on the porous composite oxide carrier, and having a magnesium content of 5 to 55% by weight, calculated as Mg, based on the weight of the catalyst, an aluminum content of 5 to 35% by weight, calculated as Al, based on the weight of the catalyst, a nickel content of 1.2 to 60% by weight, calculated as Ni, based on the weight of the catalyst, and a total amount of magnesium, aluminum and nickel of 50 to 85% by weight, calculated as a sum of Mg, Al and Ni, based on the weight of the catalyst, the fine nickel metal particles having an average particle diameter of not more than 10 nm.