An objective of the present invention is to provide ionic polymers that can provide for polymer-comprising substrates whose surfaces can stably and easily immobilize ligands such as sugar chains interacting with biological substances, in which the ligand portion exists stably even in water or buffer, and its detection capability does not decrease for a long period of time; and substrates comprising the polymers.The ionic polymer of this invention comprises two or more ionic functional groups (A) and two or more biointeracting groups represented by formula L-X-B-[wherein, L denotes a ligand capable of interacting with a biological substance, X denotes a spacer, and B denotes a binding group] as side chains covalently bound to a main chain polymer.