A lipid derivative in which a hydrophilic polymer is bound through a cyclic benzylidene acetal linker, and which can accurately control a hydrolysis rate in the weakly acidic environment of a living body to detach the hydrophilic polymer from a lipid membrane structure. The lipid derivative is represented by 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； R7 is a hydrocarbon group having from 8 to 24 carbon atoms, an acyl group having from 8 to 24 carbon atoms, a cholesterol derivative, a glycerolipid, a phospholipid or a sphingolipid； P is a hydrophilic polymer； s is 1 or 2, t is 0 or 1, and s+t is 1 or 2； and Z1 and Z2 are each independently a selected divalent spacer.
