Porous carbon electrode substrate, method of manufacturing same, gas diffusion layer, and membrane-electrode assembly for fuel cell

A porous carbon electrode substrate comprising short carbon fibers and carbonized resin bonding the short carbon fibers, the porous carbon electrode substrate having an average short circuit current value of 10 mA or less measured at a first surface A during a short circuit current test applying 1.0V of DC voltage,

• wherein,
• in three layers obtained by dividing into a trisection, a section of the porous carbon electrode substrate from a surface having a 50% filling rate closest to that of the first surface A to a surface having a 50% filling rate closest to that of a second surface in a direction perpendicular to a surface of the porous carbon electrode substrate, a layer X close to the first surface A and having a largest filling rate, a layer Y close to the second surface and having a filling rate smaller than that of the layer X, and a layer Z present between the layer X and the layer Y,
• the three lasers have filling rates in descending order of layer X, layer Y and layer Z, and
• the filling rates are obtained by three-dimensional measuring X-ray CT.