Patent ID: 8133635

Claim:
An electrolyte membrane comprising a graft polymer having a sulfonic acid group as a proton conductive group, wherein, when the electrolyte membrane is divided into four equal parts in a thickness direction thereof, a content of the sulfonic acid group in each of outer regions is larger than a content of the sulfonic acid group in each of inner regions; wherein A 1 , A 2 , B 1 and B 2 satisfy the following formula: 1.5≦( A 1 +A 2)/( B 1+ B 2)≦8 wherein A 1 and A 2 each represent a maximum value of a distribution amount of the sulfonic acid group in each of said two outer regions, and B 1 and B 2 each represent an average value of a maximum value and a minimum value of a distribution amount of the sulfonic acid group in each of said two inner regions; and wherein the electrolyte membrane has an ion-exchange capacity of 0.5 to 2 meq/g.