A material is provided for immobilization of biologically active substances which are reactive with a carbodiimide group. The material contains a carbodiimide group-containing polymer supported on a carrier such as a plastic, an inorganic polymer, a metal, a natural polymer or a ceramic. The carbodiimide group-containing polymer has 2 to 100 carbodiimide groups per molecule and a molecular weight of 1,000 to 100,000, and is prepared by carbodiimidization of an organic polyisocyanate in the presence of a catalyst. The polymer may be supported as a film on part or the whole area of the carrier. Biologically active substances that may be immobilized include enzymes, hormones, antibodies, antigens, heptenes, peptides, DNAs and RNAs.