NOVELTY - Reformer includes a heat source for evaporating a mixed fuel comprising water and fuel including hydrogen, and a reforming reactor for generating hydrogen gas from the mixed fuel. The heat source and/or the reforming reactor are formed of an alloy of a barrier layer forming metal and a mechanical strength enhancing metal.    USE - The reformer is used in a fuel cell system (e.g. polymer electrolyte membrane fuel cell) to generate a hydrogen-rich reformed gas from a mixed fuel comprising water and a fuel including hydrogen (claimed). The fuel cell system is used a power generation system for producing electrical energy.    ADVANTAGE - The reformer provides fuel cell system that has improved welding characteristics by using an alloy of a metal that is capable of forming a barrier layer and a metal that is capable of enhancing mechanical strength, and improving adhesion between a substrate and a catalyst by forming an oxide film.    DETAILED DESCRIPTION - Reformer (30) comprises a heat source (31) for evaporating a mixed fuel comprising water and fuel including hydrogen. The heat source has a heat source flow path through which the mixed fuel passes. The heat source flow path has a predetermined length and a first catalyst layer lining an internal surface of the heat source flow path. A reforming reactor (32) generates hydrogen gas from the mixed fuel. It has a reforming reactor flow path through which the mixed fuel passes. The reforming reactor flow path has a predetermined length and a reforming catalyst layer lining an internal surface of the reforming reactor flow path. The heat source and/or the reforming reactor are formed of an alloy of a barrier layer forming metal and a mechanical strength enhancing metal. An INDEPENDENT CLAIM is used for a fuel cell system (100) comprising a stack (10) for generating electricity through an electrochemical reaction of a fuel and an oxidant; an oxidant supplier for supplying air to the stack; the above reformer; and a fuel supplier () for supplying the mixed fuel to the reformer.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of the structure of a fuel cell system.    Stack (10)    Reformer (30)    Heat source (31)    Reforming reactor (32)    Carbon monoxide remover (33,34)    Fuel supplier (50)    Reformer (100)