A planar solid oxide fuel cell is investigated as an electrochemical tubular reactor with two dimensional reactive surface and a rectangular cross section. Analysis and experiments were carried out in order to clarify the prediction of electrical output, flow pattern and thermal generation. The predicted resistance of cell materials and the measured polarization show that the electrical output is expected to be 4.5 x 10(3) W/m(2) of output density and 50% of electrical conversion efficiency using a hydrogen feed. A feed gas is distributed to gas flow tubes through a gas manifold. Numerical analysis shows that each gas flow rate varies in the tubes when cell size in enlarged. The ratio of fuel partial pressure at outlet to that at inlet in a tube clearly decreases when Reynolds number in the tube increases to above 0.1 and fuel utilization to above 50%. It is clarified that the electrical power generation heated up a planar solid oxide fuel cell and the heat distributed the temperature in the cell plane.