A fuel electrode catalyst for fuel cell excellent in CO poisoning resistance, an electrode/membrane assembly using the fuel electrode catalyst for fuel cell, and a fuel cell and a fuel cell system including the electrode/membrane assembly are provided. The fuel electrode catalyst for fuel cell comprises a platinum-ruthenium first alloy catalyst and a second alloy catalyst obtained by partially substituting ruthenium of the platinum-ruthenium first alloy catalyst by a metal lower dissolving potential than ruthenium. The electrode/membrane assembly 7 comprises three layers of a second alloy catalyst layer 3, a first alloy catalyst layer 4, and a ruthenium catalyst layer 5 arranged in this order from a polymer electrolytic membrane 1 side toward a gas diffusion layer 13 side.