Thus, the present invention provides a method for producing a biosensor comprising a substrate coated with a hydrophilic polymer, which comprises allowing a compound generating a reactive group as a result of external stimulus to bind to the surface of a substrate, and then giving external stimulus in a state where a hydrophilic polymer capable of forming a covalent bond with said reactive group is allowed to come into contact with the substrate, so that said hydrophilic polymer is allowed to bind to the substrate surface via a covalent bond with said reactive group.