A unit cell of a fuel battery such that the mechanical load on the electrolyte membrane is lightened and an adequate load per unit area can be exerted on the central part. The unit cell comprises a membrane electrode assembly which has an anode electrode provided on one side of a solid polymer electrolyte film and including an anode catalyst layer and a gas diffusing layer and a cathode electrode provided on the other and including a cathode catalyst layer and a gas diffusing layer and a pair of separators. The unit cell is characterized in that at least on one side of the anode and cathode sides of the solid polymer electrolyte film, the anode or cathode catalyst layer has dimensions and a shape a size smaller than those of the solid polymer electrolyte film and the gas diffusion layers, the outer periphery of the solid polymer electrolyte film protruding from the periphery of the anode or cathode catalyst layer is opposed to the outer periphery of the gas diffusing layer, a protective layer having a picture-frame shape and having a first portion interposed between the outer periphery of the solid polymer electrolyte film and the outer periphery of the gas diffusing layer and a second portion placed on the outer periphery of the anode or cathode catalyst layer is provided, and a unit cell thickness adjustment layer is provided in which the thickness of the unit cell thickness adjustment layer in the region where the second portion of the protective layer is present is smaller than the thickness of the unit cell thickness adjustment layer in the central region where the protective layer is not present or the unit cell thickness adjustment layer is not provided so that the thickness of the unit cell in the region where the second portion of the protective layer is present may be the thickness or less of the unit cell in the central region.