NOVELTY - Polymer electrolyte fuel cell has a membrane-electrode assembly in which electrode including catalyst layer is joined on both surfaces of an electrolyte film. One or more portion of membrane-electrode assembly contains slightly soluble salt including cesium.    USE - Polymer electrolyte fuel cell used as vehicle-mounted motive power source, fixed type electric power generation device and cogeneration system. Can also be used for water electrolysis apparatus, hydrohalic acid electrolyzer, brine-electrolysis apparatus, oxygen and/or hydrogen concentrator, humidity sensor and gas sensor.    ADVANTAGE - The reduction in performance of fuel cell resulting from chloride ion is suppressed.    DETAILED DESCRIPTION - Polymer electrolyte fuel cell has a membrane-electrode assembly in which electrode including catalyst layer is joined on both surfaces of an electrolyte film. One or more portion of membrane-electrode assembly contains slightly soluble salt including cesium. The slightly soluble salt is a complex salt containing cesium, chlorine and platinum group element or complex salt containing cesium, fluorine and metallic elements other than platinum group element. The metallic elements are chosen from aluminum, gallium, chromium, copper, titanium, zirconium, cerium and/or lanthanum. The complex salt containing cesium, chlorine and platinum group element is chosen from cesium-platinum hexachloride (Cs2PtCl6), cesium-palladium dichloride (Cs2PdCl2), cesium-iridium hexachloride (Cs2IrCl6) and/or cesium-rhodium hexachloride (Cs2RhCl6). The complex salt containing cesium, fluorine and metallic elements, is chosen from cesium-aluminum pentafluoride (CsAl2F5), cesium-aluminum decafluoride (CsAl3F10), cesium-aluminum tetrafluoride (CsAlF4), cesium-aluminum hexafluoride (Cs3AlF6), cesium-aluminum tetrafluoride dihydrate (CsAlF4.2H2O), cesium-aluminum pentafluoride monohydrate (Cs2AlF5.H2O), cesium-gallium tetrafluoride (CsGaF4), cesium-gallium tetrafluoride dihydrate (CsGaF4.2H2O), cesium-chromium pentafluoride monohydrate (CsCrF5.H2O), cesium-chromium hexafluoride (CsCrF6), cesium-copper tetrafluoride (Cs2CuF4), cesium-titanium tetrafluoride (CsTiF4), cesium-titanium hexafluoride (CsTiF6) and/or cesium-cerium heptafluoride (CsCe2F7). The cesium ion exchange is carried out to a portion of proton of acid radical of the electrolyte film and/or electrolyte in catalyst layer.