A hydrophilic polymer derivative having a cyclic benzylidene acetal linker represented by the following formula (1): wherein R1 and R6 are each independently a hydrogen atom or a hydrocarbon group; R2, R3, R4 and R5 are each independently an electron-withdrawing or electron-donating substituent or a hydrogen atom; X1 is a chemically reactive functional group; P is a hydrophilic polymer, s is 1 or 2, t is 0 or 1, and s+t is 1 or 2; w is an integer of 1 to 8; and Z1 and Z2 are each independently a selected divalent spacer.