Abstract: An apparatus for an endothermic reaction of a gas such as steam reforming having a cylindrical vessel which is provided therein a fuel gas supply room, a combustion catalyst holding wall, a flue gas passage room, and a reaction room, all of which are cylindrical or annular and coaxially piled in this order, the catalyst holding wall having a combustion catalyst holding layer on a flue gas passage room side thereof and the reaction room having a product gas passage; whereby a mixture of a fuel gas and an oxygen containing gas passes through the catalyst holding wall from the supply room into the flue gas passage room, being burnt by the catalyst to become a flue gas and to generate combustion heat, and the flue gas is discharged while the heat is given to the reaction room mainly by radiation from the catalyst layer; a raw material gas passes through the reaction room reacting endothermically to become a product gas, which is discharged through the product gas passage in countercurrent heat exchange arrangemen