TECHNOLOGY FOCUS - POLYMERS - Preferred Properties: The weight fraction (Wh1) is 0.1-0.65. The weight fraction (Wh2) is 0.05-0.5. The difference of ion-exchange capacity of ionically conductive polymer (I) and ion-exchange capacity of ionically conductive polymer (II) is 0.05-1.5 meq/g. The ion-exchange capacity of ionically conductive polymer (I) and ion-exchange capacity of ionically conductive polymer (II) are 0.5-4 meq/g. The ratio of polystyrene conversion number average molecular weight calculated by gel permeation chromatography of ionically conducive polymer (I) and polystyrene conversion number average molecular weight calculated by gel permeation chromatography of ionically conducive polymer (II) is 0.1-10. The ratio of weight fraction of ionically conducive polymer (I) and weight fraction of ionically conducive polymer (II) in the polyelectrolyte composition is 95/5-5/95. Preferred Segment: The segment having ion-exchange group of ionically conductive polymer (I) and the segment having ion-exchange group of ionically conductive polymer (II) consist of mutually same structural unit. The segment which does not have ion-exchange group of ionically conductive polymer (I) and the segment which does not have ion-exchange group of ionically conductive polymer (II) consist of mutually same structural unit. The segment having ion-exchange group of ionically conductive polymer (I) and/or the segment having ion-exchange group of ionically conductive polymer (II), has an aromatic ring in the principal chain of the segment, and a side chain which has an aromatic ring. The segment having ion-exchange group of ionically conductive polymer (I) and/or ionically conductive polymer (II) is of formulae: asterisk-(Ar1-Z-Ar2-X)m-asterisk (A1), asterisk-(Ar3-Z'-Ar4-X'-Ar5(-Y'-Ar6)p-X')m-asterisk (A2), -((Ar7)q-X"(-Ar8)r-X")m-asterisk (A3) and -(Ar9)m- (A4). The segment with does not have ion-exchange group is of formulae: asterisk-(Ar11-Z-Ar12-X)n-asterisk (B1), asterisk-(Ar13-Z'-Ar14-X'-Ar15(-Y'-Ar16)s-X')n-asterisk (B2), -((Ar17)u-X"(-Ar18)t-X")n-asterisk (B3) and -(Ar19)n- (B4).m,n=5 or more;Ar1-Ar9=bivalent aromatic group optionally having side chain which has aromatic ring, and aromatic ring in principal chain, where aromatic ring of principal chain and/or aromatic ring of side chain has ion-exchange group directly bonded to aromatic ring;Z,Z'=CO or SO2;X,X',X"=oxygen or sulfur;Y'=direct bond, or group of formula: asterisk-C(-R1)(-R2)-asterisk;p,s=0, 1 or 2;q,r,u,t=1, 2 or 3;Ar11-Ar19=bivalent aromatic group optionally having substituent as side chain;andR1,R2=H, or (un)substituted (1-10C alkyl, 2-20C acyl, 6-18C aryloxy, 6-18C aryl or 1-10C alkoxy), where R1 and R2 are optionally connected to form ring.Preferred Compound: The ionically conductive polymer (I) and/or ionically conductive polymer (II) is aromatic polymer. Preferred Group: The ion-exchange group in the ionically conductive polymer (I) and/or ion-exchange group in the ionically conductive polymer (II) is a sulfonic acid group. Preferred Membrane: A polyelectrolyte membrane has a micro phase-separation structure comprising a phase in which density of the segment which has ion-exchange group is higher than the density of the segment which does not have ion-exchange group, and another phase in which density of the segment which does not have ion-exchange group is higher than the density of the segment which has ion-exchange group.