To provide a lipid derivative in which a hydrophilic polymer is bound through an acetal linker, and which can accurately control a hydrolysis rate at the pH of the weakly acidic environment in the living body to detach the hydrophilic polymer from a lipid membrane structure. A lipid derivative in which a hydrophilic polymer is bound through a cyclic benzylidene acetal linker represented by formula (1) : wherein, R 1 and R 6 are each independently a hydrogen atom or a hydrocarbon group； R 2 , R 3 , R 4 and R 5 are each independently an electron-withdrawing or electron-donating substituent or a hydrogen atom； R 7 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 Z 1 and Z 2 are each independently a selected divalent spacer.
