A polymer electrolyte membrane having outstanding water resistance and high thermal resistance, moreover having practical strength required for use as a polymer electrolyte membrane of a solid polymer electrolyte type fuel cell at low price, and a method for producing the polymer electrolyte membrane are provided. A polymer electrolyte comprising a block copolymer comprising one or more of blocks in which sulfonic acid groups are introduced and one or more blocks in which sulfonic acid groups are not substantially introduced wherein at least one block in the block copolymer is a block having aromatic rings in polymer chain, and a porous membrane, and a fuel cell using the membrane are provided.And moreover the present invention provides a method for producing a polymer electrolyte membrane comprising the steps of, (i) impregnating some of pores of a porous membrane with a solution (1) of a polymer electrolyte having a contact angle of less than 90.degree. to said porous membrane, (ii) impregnating a remaining part in the pores of the porous membrane with a solution (2) of a polymer electrolyte having a larger contact angle than that of the solution (1) to said porous membrane, and (iii) removing a solvent.