The present invention provides ink for forming a catalyst layer containing at least a cation conductive polymer electrolyte, catalyst-supporting particles including conductive carbon particles and an electrode catalyst supported thereon, and a dispersion medium, wherein the polymer electrolyte has a mean inertia radius of 150 to 300 nm. A catalyst layer made of the catalyst layer ink improves in gas diffusion property and increases cell voltage, which allows providing a proton conductive polymer electrolyte fuel cell capable of maintaining the high cell voltage for a long time.