The present invention aims at providing a catalyst as a porous catalyst body for decomposing hydrocarbons which comprises at least magnesium, aluminum and nickel, wherein the catalyst has an excellent catalytic activity for decomposition and removal of hydrocarbons, an excellent anti-sulfur poisoning property, an excellent anti-coking property even under a low-steam condition, a sufficient strength capable of withstanding crushing and breakage even when coking occurs within the catalyst, and an excellent durability. The above aim of the present invention can be achieved by a porous catalyst body for decomposing hydrocarbons which comprises at least magnesium, aluminum and nickel in such a manner that the magnesium and aluminum are present in the form of a composite oxide of magnesium and aluminum, and the nickel is present in the form of metallic nickel; and which porous catalyst body has a magnesium element content of 10 to 50% by weight, an aluminum element content of 5 to 35% by weight and a nickel element content of 0.1 to 30% by weight, a pore volume of 0.01 to 0.5 cm3/g, an average pore diameter of not more than 300 Å and an average crushing strength of not less than 3 kgf.