A porous catalyst body for decomposing hydrocarbons which consists essentially of 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, the metallic nickel having an average particle diameter of 0.5 to 20 nm; 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 cm 3 /g, an average pore diameter of not more than 300 Å, an average crushing strength of from 3 to 50 kgf, and a BET specific surface area of 10 to 100 m 2 /g, the crushing strength being determined from an average value of strengths of 100 catalyst moulded products which were measured using a digital force gauge manufactured by Imada K.K. according to JIS Z 8841, said porous catalyst body being obtainable by a method in which the respective constitutional elements are subjected to a co-precipitation reaction to form hydrotalcite particles as a precursor, then the resulting hydrotalcite particles are mixed with a molding assistant and a binder and further with water and an alcohol as a dispersing medium, and the resulting mixture is kneaded using a kneader to form a clayey mass, followed by molding the resulting clayey mass, which is then dried and further heat-treated at a temperature of 700 to 1500°C for a time is 1 to 72 hours and then subjected to reduction treatment at a temperature of 700 to 1000°C for a time of 0.5 to 24 hours, the molding assistant being added in an amount of 1 to 50 parts by weight based on 100 parts by weight of the hydrotalcite compound particles consisting of magnesium, aluminum and nickel as metals and the binder being added in an amount of 1 to 50 parts by weight based on 100 parts by weight of the hydrotalcite compound particles consisting of magnesium, aluminum and nickel as metals, the molding assistant being one or more selected from celluloses, polyvinyl alcohol, starches, methyl cellulose, maltose and carboxymethyl cellulose, the binder being one or more selected from α-alumina, an aluminum salt, silica, clay, talc, bentonite, zeolite, cordierite, a titania alkali metal salt, an alkali earth metal salt, a rare earth metal salt, zirconia, mullite, sepiolite, montmorillonite, halloysite, saponite, stevensite, hectorite, and silica alumina. said porous catalyst body being obtainable by a method in which the respective constitutional elements are subjected to a co-precipitation reaction to form hydrotalcite particles as a precursor, then the resulting hydrotalcite particles are mixed with a molding assistant and a binder and further with water and an alcohol as a dispersing medium, and the resulting mixture is kneaded using a kneader to form a clayey mass, followed by molding the resulting clayey mass, which is then dried and further heat-treated at a temperature of 700 to 1500°C for a time is 1 to 72 hours and then subjected to reduction treatment at a temperature of 700 to 1000°C for a time of 0.5 to 24 hours, the molding assistant being added in an amount of 1 to 50 parts by weight based on 100 parts by weight of the hydrotalcite compound particles consisting of magnesium, aluminum and nickel as metals and the binder being added in an amount of 1 to 50 parts by weight based on 100 parts by weight of the hydrotalcite compound particles consisting of magnesium, aluminum and nickel as metals, the molding assistant being one or more selected from celluloses, polyvinyl alcohol, starches, methyl cellulose, maltose and carboxymethyl cellulose, the binder being one or more selected from α-alumina, an aluminum salt, silica, clay, talc, bentonite, zeolite, cordierite, a titania alkali metal salt, an alkali earth metal salt, a rare earth metal salt, zirconia, mullite, sepiolite, montmorillonite, halloysite, saponite, stevensite, hectorite, and silica alumina.