To provide a hydrophilic polymer derivative having a novel linker technique which is able to from a hydrophilic polymer pro-drug of a drug containing an amino group or the like in order to overcome the restrictions of conventional hydrophilic polymer pro-drugs. A hydrophilic polymer derivative containing a hydrophilic polymer moiety and an acetal moiety, wherein the hydrophilic polymer derivative contains a structure represented by formula (1) or formula (2):
