A catalyst layer for polymer electrolyte fuel cells that improves drainage or gas diffusion, reduces or prevents the occurrence of cracking in a catalyst layer, enhances catalyst utilization efficiency, exerts high output power and high energy conversion efficiency, and has high durability, and also provides a membrane-electrode assembly and a polymer electrolyte fuel cell using the catalyst layer. The catalyst layer for polymer electrolyte fuel cells contains a catalyst, carbon particles, a polymer electrolyte, and a fibrous material. In the catalyst layer, the carbon particles carry the catalyst1. The catalyst layer for polymer electrolyte fuel cells has voids. The percentage of frequencies of the voids having a cross-sectional area of 10,000 nm.sup.2 or more is 13% or more and 20% or less among the voids observed in a thickness-direction cross section of the catalyst layer for polymer electrolyte fuel cells perpendicular to the surface thereof.

A fuel cell module includes a first fuel cell layer and second fuel cell layer. Each fuel cell layer includes a plurality of membrane electrode assemblies arranged in a planar shape. Each membrane electrode assembly includes an electrolyte membrane, an anode disposed on one surface of the electrolyte membrane, and a cathode disposed on the other surface of the electrolyte membrane. Each fuel cell layer also includes interconnectors each of which electrically connects the anode of one of adjacent membrane electrode assemblies to the cathode of the other. The first fuel cell layer and second fuel cell layer are arranged so that one polarity of each membrane electrode assembly of the first fuel cell layer is opposed to the same polarity of each membrane electrode assembly of the second fuel cell layer.

A fuel cell module includes a first fuel cell layer and second fuel cell layer. Each fuel cell layer includes a plurality of membrane electrode assemblies arranged in a planar shape. Each membrane electrode assembly includes an electrolyte membrane, an anode disposed on one surface of the electrolyte membrane, and a cathode disposed on the other surface of the electrolyte membrane. Each fuel cell layer also includes interconnectors each of which electrically connects the anode of one of adjacent membrane electrode assemblies to the cathode of the other. The first fuel cell layer and second fuel cell layer are arranged so that one polarity of each membrane electrode assembly of the first fuel cell layer is opposed to the same polarity of each membrane electrode assembly of the second fuel cell layer.