A reformer 1 of the present invention is formed by stacking each other a reforming layer 2 configured to reform a reforming fuel with a reforming catalyst 5, and a heating layer 3 configured to heat the reforming layer 2 through an exothermic reaction with a combustion catalyst 8. A gas mixture obtained by mixing a to-be-combusted gas with a combustion-assisting gas is supplied to a heating passage 25 in the heating layer 3. The heating passage 25 is separated into multiple separation passages 31a to 31c by separation walls 11, 13 parallel to a wall surface coated with the combustion catalyst 8. The multiple separation passages 31a to 31c respectively eject the gas mixture to different positions of the combustion catalyst 8.