NOVELTY - A direct oxidation fuel cell comprises anode-side separator (14) having fuel flow channel (20) for supplying fuel to anode; and cathode-side separator (15) having oxidant flow channel (21) for supplying oxidant to cathode, where anode includes anode catalyst layer (16) and anode diffusion layer (17) comprising conductive water repellent layer disposed at side of anode catalyst layer and includes conductive agent and water repellent agent, and substrate layer disposed at side of separator, where porosity of substrate layer is higher at downstream side of fuel flow.    USE - As direct oxidation fuel cell (claimed) useful as power source for compact devices such as cell phones, laptop computers, and digital cameras.    ADVANTAGE - The direct oxidation fuel cell has reduced methanol crossover (MCO) and excellent long life characteristics even when using an aqueous methanol solution containing a high concentration of methanol which is highly diffusive, by controlling fuel diffusivity in the anode. In the fuel cell, MCO can be reduced at the upstream side of the fuel flow and the supply amount of methanol can be secured at the downstream side. Since the output power degradation due to MCO and the output power degradation due to supply amount deficiency of methanol can both be controlled, long life characteristics of the fuel cell improve significantly. In addition, control of fuel diffusivity is easier when controlled at the substrate layer than at the conductive water repellent layer. Therefore, methanol diffusivity can be controlled at the upstream side and the downstream side of the fuel flow, even when using an aqueous methanol solution containing a high concentration of methanol which is highly diffusive. The use of an aqueous methanol solution with high concentration methanol enables the fuel cell to be lighter or smaller.    DETAILED DESCRIPTION - A direct oxidation fuel cell comprises: a membrane electrode assembly (13) including an anode (11), a cathode (12), and an electrolyte membrane (10) interposed between the anode and cathode; an anode-side separator (14) having a fuel flow channel (20) for supplying fuel to anode; and a cathode-side separator (15) having an oxidant flow channel (21) for supplying oxidant to cathode, where anode includes an anode catalyst layer (16) disposed at the side of electrolyte membrane and an anode diffusion layer (17) disposed at the side of anode-side separator, where the anode diffusion layer includes: a conductive water repellent layer disposed at the side of anode catalyst layer and including a first conductive agent and a first water repellent agent; and a substrate layer disposed at the side of anode-side separator; where the porosity of substrate layer is higher at the downstream side than at the upstream side of the fuel flow.    DESCRIPTION OF DRAWING(S) - The figure shows a vertical sectional view schematically illustrating an example of a direct oxidation fuel cell.    Electrolyte membrane (10)    Anode (11)    Cathode (12)    Membrane electrode assembly (13)    Anode-side separator (14)    Cathode side separator (15)    Fuel flow channel (20)    Oxidant flow channel (21)