To provide a biodegradable hydrogel having an acetal structure whose hydrolysis rate under different pH environments in the living body can be accurately controlled. A hydrogel obtained by crosslinking a polyalkylene glycol derivative having a cyclic benzylidene acetal structure represented by formula (1) shown below with a crosslinking agent. In formula (1), 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； s is 1 or 2, t is 0 or 1, and s + t is 1 or 2； P 1 is a polyalkylene glycol having the number of terminals from 2 to 8； Z 1 and Z 2 are each independently a selected divalent spacer； W 1 is an integer of 2 to 8 and is equal to the number of terminals of the polyalkylene glycol； and X 1 is a chemically reactive functional group.
