In a conventional polymer membrane electrode assembly, particularly when operated for a long period of time, a portion of the polymer electrolyte membrane to be in contact with the gas diffusion layer has suffered significant degradation. In order to address this, in a membrane electrode assembly including a hydrogen ion conductive polymer electrolyte membrane, a pair of catalyst layers arranged on both surfaces of the polymer electrolyte membrane, and a pair of gas diffusion layers, each including a fibrous substrate, arranged on the outer surfaces of the catalyst layers, a thickness T.sub.A of a center portion that faces the catalyst layer and a thickness T.sub.B of a peripheral portion surrounding the center portion are set to satisfy a expression (1): 0.7.ltoreq.T.sub.B/T.sub.A.ltoreq.0.9.