An electrolyte membrane/electrode structure constituting a fuel cell comprises a solid polymer electrolyte membrane, an anode side electrode and a cathode side electrode sandwiching the solid polymer electrolyte membrane. The anode side electrode is provided with an electrode catalyst layer and a gas diffusion layer abutting on one side of the solid polymer electrolyte membrane and exposing the outer circumference thereof in the shape of a frame, and the cathode side electrode is provided with an electrode catalyst layer and a gas diffusion layer abutting on the other side of the solid polymer electrolyte membrane. A reinforcing sheet member is arranged on the frame-shaped surface of the solid polymer electrolyte membrane projecting from the outer circumference of the gas diffusion layer.