Abstract: A method for producing a terminal carboxyl group-containing polyethylene glycol, which includes the following steps 1 and 2. Step 1: In a solution containing a polyethylene glycol having a hydroxyl group, an inorganic base 0.5 to 4 times the polyethylene glycol in mass, an aprotic solvent and water, mass of the water being 5 to 20 meq with respect to the inorganic base and mass of the water in the solution being 2,000 ppm or less, a compound of the formula (1) is reacted with the polyethylene glycol to obtain a polyethylene glycol ester: wherein X represents a leaving group, a represents an integer of 4 to 9, and R1 represents a hydrocarbon group having 1 to 4 carbon atoms; Step 2: The polyethylene glycol ester is hydrolyzed to obtain a terminal carboxyl group-containing polyethylene glycol.