An object of the present invention is to provide a biosensor comprising a hydrogel capable of immobilizing a physiologically active substance thereon, which can be produced conveniently by use of a safe material, and a method for producing the same. The present invention provides a method for producing a biosensor, which comprises bringing a polymer containing an activated carboxyl group into contact with a substrate surface coated with an organic layer having an amino group to thereby bind the polymer to the organic layer.