A bonded membrane-electrode assembly is provided which enables the electrolysis of water, which has water-electrolyzing and power-generating functions enabling the generation of hydrogen, and which is capable of being utilized for a fuel cell; and a fuel cell system with a water electrolyzer is provided using a plurality of such bonded membrane-electrode assemblies. The bonded membrane-electrode assembly includes a solid polymer electrolyte membrane, an oxygen electrode bonded to one of sides of the solid polymer electrolyte membrane, and a hydrogen electrode bonded to the other side of the solid polymer electrolyte membrane. The oxygen electrode includes a porous sheet-shaped carbon element plated with iridium, a first coated layer formed on a surface of the sheet-shaped carbon element, which is in contact with the solid polymer electrolyte membrane, by applying a mixture containing carbon and a resin for the solid polymer electrolyte membrane on that surface, and a second coated layer formed on the first coated layer by applying a mixture containing Pt (alloy) and/or Pt (alloy)-carried carbon and a resin for the solid polymer electrolyte membrane thereon. The hydrogen electrode includes a porous sheet-shaped carbon element, a first coated layer formed on the sheet-shaped carbon element by applying a mixture containing carbon and a resin for the solid polymer electrolyte membrane on the surface thereof, and a second coated layer formed on the first coated layer by applying a mixture containing Pt (alloy) and/or Pt (alloy)-carried carbon and a resin for the solid polymer electrolyte membrane thereon.