Methotrexate, which is a known anticancer agent, can be converted into a polymeric conjugate with a pyran copolymer, which in itself has a certain anticancer effect, by a method in which an alkylene diamine is reacted with the carboxyl group of the methotrexate molecule to give an amidation product having a free amino group in a molecule, which is then reacted with the carboxyl group of the pyran copolymer, of which at least 10% of the repeating units have been modified with the methotrexate residue. The polymeric conjugate has sustained releasability of the methotrexate compound in blood serum so that an excellent therapeutic effect can be obtained by the administration thereof against cancers with little problems due to the side effects caused by an unduly high concentration of the drug in blood unavoidable when methotrexate as such is administrated.