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), R 1 -R 6 are hydrogen atoms or the like; s = 1 or 2, t = 0 or 1, and s + t = 1 or 2; P 1 and P 2 are polyethylene glycol having three or more ethylene glycol units; w, u, and v are integers of 1 or the like; X 1 is a functional group capable of chemical reaction; and Z 1 , Z 2 , and Z 3 are divalent spacers.)