A catalytic reactor requires an effective exchange of heat energy, a quick load response and a downsized dimension, as is seen for the reformer system that generates hydrogen for fuel cells. A wall-type reactor, which metallic wall is directly catalyzed, is attracting interest as a reactor that would satisfies such demands. This research studied the performance of reaction and heat transfer of a rectangular wall reactor, for decomposing methanol to hydrogen and carbon monoxide. The reactor consists of alternating reaction channels with a plate-fin type nickel catalyst prepared by electroless plating between heat medium channels. Furthermore, the dynamic response of the reactor was also examined when the flow rate of feed gas was rapidly changed. The temperature profiles in the rectangular wall reactor demonstrated that the reactor effectively supplies heat energy to reaction zone, even under the reaction condition with a large amount of energy consumption. In addition, the performance of the reactor to handle the feeding material depended on the channel height and the shape of the plate fins inserted into the reaction channel. The performance increased as the height decreased and when serrated-type fins were used. The overall coefficient of heat transfer estimated from the temperature distribution in the reactor suggests that the performance increased because the heat conductivity of the reactor improved as a result of changing the channel height. The heat conductivity of effluent gas stabilized within a few seconds from when the flow rate of feed gas was changed instantaneously. The time required for the reaction to attain steady state was short. The results suggest that the constructed reactor responds to load fluctuation quickly.