[Problems] A derivative of a nucleic acid antimetabolite is demanded which can show a high therapeutic effect at a low dose. [Means For Solving Problems] Disclosed is a high molecular weight derivative of 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 carboxyl group in the side chain is bound to a hydroxyl group in the nucleoside derivative via an ester bond. Also disclosed is a method for producing the high molecular weight derivative.