There are provided a new crosslinked polymer electrolyte excellent in water resistance and solvent resistance, high in heat resistance, inexpensive and low in methanol permeability, and suitable for the proton conductive membrane of a fuel cell, by means of the crosslinked polymer electrolyte obtained by the following (1) or (2), and its production method.(1) A compound having two or three or more reactive groups is reacted with a polymer electrolyte.(2) A compound having two or three or more reactive groups is reacted with a polymer to obtain a crosslinked polymer and then an ion exchange group is introduced into the resultant polymer.