A functional membrane and a production method thereof including: an ion irradiation step in which a polymer film substrate is irradiated with high energy heavy ions at 10 4 to 10 14 ions/cm2, to generate active species in the film substrate; and a graft polymerization step in which after the ion irradiation step, the film substrate is added with one or more monomers selected from a group A consisting of monomers each having a functional group and 1 to 80 mol % of a monomer including a group B consisting of a crosslinking agent(s) for the group A monomer(s), and the film substrate and the monomer(s) are graft-polymerized. There is obtained a functional membrane having high functionality in conjunction with the gas barrier property intrinsically possessed by a polymer film substrate, in particular, a polymer electrolyte membrane optimal as a polymer electrolyte membrane for use in fuel cells, high in proton conductivity and excellent in gas barrier property.