Provided is a high-molecular-weight branched, degradable polyethylene glycol derivative which does not cause cell vacuoles. The branched, degradable polyethylene glycol derivative represented by formula (1) includes an oligopeptide that decomposes intracellularly in each molecule. (In the formula, k1 and k2 are independently 1–12, j1 and j2 are independently 45–950, R is a hydrogen atom, a substituted or unsubstituted C1-12 alkyl group, a substituted aryl group, an aralkyl group, or a heteroalkyl group, Z is an oligopeptide which is degraded by intracellular enzymes, X is a functional group which can react with a biological substance, and L1 and L2 are independently single bonds or divalent spacers.)