A gas diffusion layer including an electroconductive porous material and 16-55% by weight of fluororesin added to the electroconductive porous material is used for at least one of the positive electrode and the negative electrode of a membrane/electrodes assembly of a polymer electrolyte fuel cell. As a result, the water-retaining property of the inside of the membrane/electrodes assembly is improved without hindering gas diffusion, thus enabling polymer electrolyte to be moistened with water formed at the positive electrode, and thereby providing a polymer electrolyte fuel cell which operates by using unhumidified gas.