A reformer includes two or more catalyst units in the flow path, capable of generating hydrogen from a reactant fluid containing an organic compound or carbon monoxide, by catalysis. In the reformer, at least two of the catalyst units satisfy the following relationship: Heat capacity of the upstream-side catalyst unit≦Heat capacity of the downstream-side catalyst unit. The reformer is improved in the relationship between these catalyst units with respect to heat capacity. This improvement leads to improvement of the reformer in safety, heating characteristics during the start-up period, efficiency of hydrogen production and reduction in production of CO as the coproduct.