Disclosed is a sulfonated multiblock copolymer, which comprises a
hydrophilic block (X) having a repeating unit represented by the
following formula 1a, and a hydrophobic block (Y) having a repeating unit
represented by the following formula 2, wherein the number (m) of the
repeating unit of formula 1a in the hydrophilic block (X) and the number
(n) of the repeating unit of formula 2 in the hydrophobic block (Y)
satisfy the conditions of 4.ltoreq.m.ltoreq.400 and
4.gtoreq.n.gtoreq.400. An electrolyte membrane obtained from the
sulfonated multiblock copolymer and a fuel cell using the electrolyte
membrane are also disclosed:Ar.sup.1--O--Ar.sup.2a--O [Formula
1a]Ar.sup.1--O--Ar.sup.3--O [Formula 2]