A method for purifying a branched polyethylene glycol, which includes adding, to a branched polyethylene glycol compound represented by the following formula [1] and having a weight average molecular weight of 40000 or more: where Z, Y1, Y2, A, Polymer, l, m and b are as defined herein, at least one solid acid selected from magnesium silicate, aluminum silicate, and aluminum magnesium silicate and having a specific surface area of 50 to 250 m2/g in the presence of an aprotic organic solvent to obtain a mixture; stirring the mixture; and subsequently separating the solid acid.