To provide a reformer 1 having improved durability, heat efficiency and reliability by using a specified structural material as a structural member of the reformer for obtaining hydrogen used for a fuel cell system by the reaction of a gaseous starting material with steam to reform.

The reformer 1 with reduced production cost and improved heat efficiency, durability and reliability is structured so that a heating tube 3 for giving the heat of reaction to a reforming catalyst 6 has an upper part structural material 21 having a projected curved surface and a U-shaped cross- section in the flow-in side of the reforming gaseous starting material and the upper structural material 21 is made from a stainless steel material excellent in the resistance against carburization.