ELECTRODE CATALYST LAYER, MEMBRANE ELECTRODE ASSEMBLY, AND POLYMER ELECTROLYTE FUEL CELL

An electrode catalyst layer that is used in a polymer electrolyte fuel cell comprises: a catalyst material; a conductive carrier that supports the catalyst material; a polymer electrolyte; and a fibrous material containing nitrogen atoms. The ratio of the number of nitrogen atoms to the total number of atoms of carbon, nitrogen, oxygen, fluorine, sulfur, and platinum obtained by energy-dispersive X-ray spectroscopy on a specific region of a cross-section of the electrode catalyst layer is 2 at % or more and 20 at % or less. The specific region is a region containing 50 area % or more of the fibrous material containing nitrogen atoms and does not contain the catalyst material or the conductive carrier.