To provide a novel polymer derivative of a cytidine metabolism antagonist with which drug release is possible independent of a biological enzyme and with which a highly therapeutic effect is expected. A polymer derivative of a cytidine metabolism antagonist in which substituent groups represented by general formula (I) or general formula (II) [where R7 and R8 are each independently a hydrogen atom or C1-6 alkyl group optionally having substituent groups； R6 is a hydrogen atom, C1-40 alkyl group optionally having substituent groups, C1-40 aralkyl group optionally having substituent groups, aromatic group optionally having substituent groups, amino acid residue protected by carboxy groups, or sugar residue optionally having substituent groups； CX-CY is CH-CH or C=C (double bond)； and A is a residue in which the amino group at position 4 has been removed from the cytidine metabolism antagonist] are bonded to the side chain carboxy groups of a block copolymer formed by a polyethylene glycol structural segment and a polymer having ten or more carboxy groups.本發明提供一種不依賴生物之酶便可進行藥劑釋放，並且期待較高之治療效果的胞核苷系代謝拮抗劑之新穎之高分子衍生物。本發明之胞核苷系代謝拮抗劑之高分子衍生物係於聚乙二醇結構部分與具有10個以上羧基之聚合物之嵌段共聚物之側鏈羧基上鍵結有通式(I)或通式(II)所示之取代基：[式中，R7、R8分別獨立地表示氫原子或可具有取代基之(C1~C6)烷基，R6表示氫原子、可具有取代基之(C1~C40)烷基、可具有取代基之(C1~C40)芳烷基、可具有取代基之芳香族基、羧基經保護之胺基酸殘基或可具有取代基之糖殘基，CX-CY表示CH-CH或C=C(雙鍵)，A表示胞核苷系代謝拮抗劑之4位胺基去除後之殘基]。
