High molecular weight derivative of nucleic acid antimetabolite

[Problems] A derivative of a nucleic acid antimetabolite is demanded which can show a higher therapeutic effect at a lower dose.[Means for Solving Problems] Disclosed is a high molecular weight derivative a nucleic acid antimetabolite, which is characterized by comprising a high molecular weight compound comprising a polyethylene glycol moiety and a polymer moiety having a carboxyl group in a side chain and a nucleoside derivative which can act as a nucleic acid antimetabolite, wherein the nucleoside derivative is bound to the carboxyl group in the side chain of the high molecular weight compound via a highly hydrophobic linker.