NOVELTY - Reformer for hydrogen generation, suitably used in industries and vehicles.    USE - Used to generate hydrogen in industries and vehicles.    ADVANTAGE - The reactor can, in a short time, generate high-purity hydrogen for a fuel cell. This generates small amounts of pollutant gasses and so is environmentally more friendly.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are made for the following:    (1) A reformer (I) dispersed in the flow path of a reactant fluid, which comprises:    (i) an electrically heatable unit of honeycomb structure, in the upstream of the flow path of a reactant fluid; and    (ii) at least one catalyst unit of honeycomb structure capable of generating hydrogen from a reactant fluid containing an organic compound or carbon monoxide, by catalysis, in the downstream of the heater unit. The heater unit and at least one of the catalyst units satisfy: Cell density of the heater unit at most Cell density of the catalyst unit.    (2) A reformer (II) as in (1) but which comprises:    (i) two or more electrical heatable units of honeycomb structure; and    (ii) at least one catalyst unit of honeycomb structure, at least at one position between the above heater units. wherein, at least one of the heater units upstream of the catalyst unit (upstream-side heater unit) and at least one of the heater units downstream of the catalyst unit (downstream-side heater unit) satisfy: Cell density of the upstream-side heater unit at least Cell density of the downstream-side heater unit.    (3) A reformer (III) as for (I) and (II) but which comprises:    (i) two or more electrically heatable heater units of honeycomb structure, adjacent to each other, installed in the direction of fluid flow; and    (ii) at least one catalyst unit of honeycomb structure as above, wherein at least two adjacent heater units satisfy the following relationship: Cell density of the upstream-side heater unit at most Cell density of the downstream-side heater unit.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic sectional view of one of the reformers.    heater unit (1)    catalyst unit (2)    metallic casing (3)    electrodes (4)    reactant fluid (A)    inlet (5)    outlet (6)    Fluid containing the produced hydrogen (B)