CATALYST LAYER, MEMBRANE-ELECTRODE ASSEMBLY, AND SOLID POLYMER FUEL CELL

A catalyst layer of the present invention is a catalyst layer comprising an interface to a polyelectrolyte membrane. The catalyst layer includes a layer forming material, which includes a catalytic substance, a conductive carrier which supports the catalytic substance, a polyelectrolyte, and a fibrous material, and a plurality of pores which contain no layer forming material. A pore area ratio which is a total area ratio of the plurality of pores to an area of a cross-section orthogonal to the interface is 25.0% or more and 35.0% or less in a cross-sectional image captured by a scanning electron microscope.