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

A catalyst layer including: a catalyst-supported carbon including a catalyst including platinum supported on a carbon carrier; and an ionomer, in which the catalyst-supported carbon has a mesopore having a pore diameter of from 2 nm to less than 10 nm in a pore distribution obtained by a nitrogen adsorption method, at least a part of the ionomer exists in the mesopore having a pore diameter of from 2 nm to less than 10 nm, a content of the ionomer with respect to 100 parts by mass of the carbon carrier is 100 parts by mass or more, and an occupancy rate of the ionomer in a total volume of the mesopore having a pore diameter of from 2 nm to less than 10 nm is 50% by volume or less.