The present invention relates to a polymer electrolyte that provides high proton conductivity and low fuel crossover at the same time, as well as a member using the same. The embodiments of the invention can achieve high output and high energy density in the form of a polymer electrolyte fuel cell. A polymer electrolyte comprising a proton conductive polymer (A) and a polymer (B) which is different from (A) wherein a ratio of the amount of unfreezable water, represented by formula (S1), in said polymer electrolyte is no less than 40 wt % and no greater than 100 wt % is disclosed. The ratio of amount of unfreezable water (S1)=(amount of unfreezable water)/(amount of low melting point water+amount of unfreezable water)×100 (%).