Providing a method for producing a compound represented by formula (1) from a compound represented by formula (2), by which the additive amount of an aryl (C1-C8) alkyl alcohol which may have a substituent and the amount of residual carboxyl groups of pAsp (polyaspartic acid) are controlled.A production method including obtaining a compound represented by formula (1) by allowing a compound represented by formula (2) to react with an aryl (C1-C8) alkyl alcohol which may have a substituent, and a carbodiimide-based compound in an amount of 2(x+y) equivalents or more relative to the amount of carboxyl groups in formula (2) (sum of x and y), in a solvent at 15° C. to 30° C. for 2 hours to 48 hours, is used.