Provided is a biodegradable polyethylene glycol derivative in which polyethylene glycol chains are linked by acetal linkers having a precisely controllable hydrolysis rate in environments of different pH within the body, and in which it is possible to precisely control the rate of splitting into low-molecular-weight polyethylene glycol chains within the body. The present invention is shown by formula (1) or formula (2). (In formula (1) and formula (2), R1-R6 are hydrogen atoms or the like s = 1 or 2, t = 0 or 1, and s + t = 1 or 2 P1 and P2 are polyethylene glycol having three or more ethylene glycol units w, u, and v are integers of 1 or the like X1 is a functional group capable of chemical reaction and Z1, Z2, and Z3 are divalent spacers.)
