TECHNOLOGY FOCUS - CHEMICAL ENGINEERING - Preferred Features:In reformer (I) the heater unit and catalyst unit next to it (and additionally all the catalyst units downstream of the heater) satisfy the following relationship when two or more catalyst units are produced:Cell density of the heater at most Cell density of the catalyst unit.In reformer (II) the upstream-side heater unit and the downstream-side heater unit with no constitutional element except one or more catalyst units in-between satisfy the relationship:Cell density of the upstream-side heater unit at least Cell density of the downstream-side heater unit.Also the upstream-side and downstream-side heater units with respect to the most upstream catalyst unit satisfy the following relationship (and additionally all of the upstream-side and downstream-side heater units), when three or more of the heater units are used with each of the catalyst units in-between at different positions:Cell density of the upstream-side heater unit at least Cell density of the downstream-side heater unit.In reformer (III) the heater unit at the most upstream position and the heater unit next to it satisfy the relationship, when three or more of the adjacent heater units are installed in the direction of the fluid flow:Cell density of the upstream-side heater unit at most Cell density of the downstream-side heater unit.Additionally all of the adjacent heater units satisfy the relationship, when three or more of the adjacent units are installed in the direction of fluid flow.Preferred Catalyst Unit: The catalyst unit contains one or more of the catalyst components for steam reforming, partial oxidation or decomposition, and/or CO shift reaction, and/or selective CO oxidation. The major ingredients are a heat-resistant oxide and at least one kind of metal element selected from the group consisting of those groups VB to VIII, IB and IIB. Preferably the metal element is group VIII, V, Cr, Mo, W, Re, metal elements of group IB and zinc.The heat resistant oxide is selected from the group consisting of Al2O3, SiO2, TiO2, ZrO2, MgO, zeolite, silicaaluminophosphate, aluminophosphate and a layer structure compound and compound oxide of it.Preferred Heating unit: The heating unit is composed of a sintered or metallic material having electrical resistance-heatability or a composite of them both or a composite of a heat-resistant material having no electrical resistance-heatability and the sintered or metallic material. EXAMPLE - Production of the heater units:    Powdered Fe, Cr-30Ai (wt.%), Fe-50AI(Wt.%) Fe-20B(wt.%) and Y2O3, all having an average particle size of 44 microns or less, were mixed to have a composition of Fe16Cr8Al0.05B0.5Y2O3 and 100g of the mixture was incorporated with 4g of methyl cellulose as the organic binder and 1 g of oleic acid as the oxidation inhibitor. The mixture thus prepared was extruded into a cylindrical honeycomb structure, which was dried at 11500C for 30 minutes in air.