A method for manufacturing a porous electrode substrate, comprising: step (1) of dispersing the pieces of a short carbon fiber (A) and a fiber (b) containing a polymer having a softening point of 250° or higher but lower than 400° C. and a particulate substance having a melting point of 400° C. or higher, in a planar direction, and thereby obtaining a precursor sheet; step (2) of impregnating the precursor sheet with a carbon powder (C2) containing powdered carbon, and a fluorine-based resin containing elemental fluorine and a resin component; and step (3) of heat treating the impregnated precursor sheet at a temperature of 250° C. or higher but lower than 400° C. in the presence of oxygen.