The present invention is characterized in that a base material comprises ester groups in the main chain and/or in the side chain(s), and comprises a polymer having hydrophobic groups. Particularly, the hydrophobic groups can be introduced by irradiating the base material with a radiation during the base material contacts an aqueous solution of a monohydric alcohol or an aqueous solution of an alcohol having not less than 2 hydroxyl groups, which is a monomer or a polymer, and which has one or more carbon atoms between the carbon atoms to each of which the hydroxyl group is bound in the monomer or in each monomer constituting the polymer.
