A porous electrode substrate in which carbon fibers are dispersed in the structure thereof have a fiber diameter of 3ˆ¼15 micron and a fiber length of 2ˆ¼30 mm, and are bound to one another by carbonized resin, wherein the pore distribution satisfies the following conditions when measuring the porous electrode substrate using mercury intrusion porosimeter. (Conditions) A pore distribution curve in which the horizontal axis is a common logarithmic scale, wherein the diameter interval of 1ˆ¼100 micron comprises 80 or more measurement points at equal intervals along the common logarithmic scale, and the skewness S of the pore distribution in the diameter interval of 1ˆ¼100 micron is -2.0