The performance of passive direct methanol fuel cells employing porous plates in the anode backing was investigated under closed circuit conditions to clarify the mechanism of methanol diffusion through the porous plate under closed circuit conditions. Three kinds of porous plates were tested to clarify the effect of porosity and water absorptivity on the steady current density using various methanol concentrations. Under the limiting current conditions, where the steady current density was proportional to the methanol concentration, a porous plate with lower porosity lowered the stable current. density at a certain methanol concentration. However, the maximum steady current density was hardly changed as compared to that for the case without a porous plate. The effective diffusion coefficient of methanol through the porous plate showed direct proportion to the porosity of the porous plate, suggesting that the entire pores in the plate were filled with CO2 gas produced at the anode surface, through which the methanol diffused as a vapor from the reservoir to the anode.