One object of the present invention is to provide a biosensor and a production method therefor, by which hydrogel that enables immobilization of a physiologically active substance can be conveniently produced using safe raw materials. The present invention provides a biosensor which comprises a substrate having a metal layer on its surface, wherein a hydrophilic polymer having a reactive functional group capable of reacting with a hydroxyl group or an amino group of a physiologically active substance is bound to the metal layer directly or indirectly via an intermediate layer.