To provide a degradable polyethylene glycol derivative in which polyethylene glycol chains are linked by a disulfide linker capable of accurately controlling the degradation rate under different reductive environments in the living body, and whose division rate into a polyethylene glycol chain of low molecular weight in the living body is able to be accurately controlled. A degradable polyethylene glycol derivative represented by formula (1). R1, R2, R3 and R4 represent each independently a hydrogen atom or a hydrocarbon group having from 1 to 6 carbon atoms, and at least one of R1, R2, R3 and R4 is the hydrocarbon group; P1 is a straight-chain or branched polyethylene glycol chain having a number of ethylene glycol units of 3 or more; P2 is a straight-chain polyethylene glycol chain having a number of ethylene glycol units of 3 or more; w is an integer of 1 to 8; u is an integer of 1 to 10; X1 is a chemically reactive functional group; and Z1, Z2 and Z3 are each independently a selected divalent spacer.