Provided are: a porous electrode substrate which has excellent handling properties and surface smoothness and satisfactory gas permeability and electrical conductivity, and enables the reduction of damage to a polymer electrolyte membrane when integrated into a fuel cell; and a process for producing the porous electrode substrate. Specifically provided are: a porous electrode substrate comprising a three-dimensional structure (Y-1) produced by bonding short carbon fibers through carbon and a three-dimensional structure (Y-2) produced by bonding short carbon fibers through carbon, wherein the three-dimensional structures (Y-1) and (Y-2) are layer stacked on and integrated with each other, the short carbon fibers form a three-dimensional entangled structure in the structure (Y-1), and the short carbon fibers do not form a three-dimensional entangled structure in the structure (Y-2); a process for producing the electrode base material; a precursor sheet for producing the electrode base material; a membrane-electrode assembly which involves the electrode base material; and a polymer electrolyte fuel cell.