The invention relates to a method for manufacturing a polyoxyethylene alkyl ether acetic acid, including feeding oxygen into a liquid phase containing polyoxyethylene ether having a hydrocarbon group at the end (hereinafter, it may also be referred to as a polyoxyethylene alkyl ether) and water, and dehydrogenating and oxidizing the polyoxyethylene alkyl ether in the presence of a platinum catalyst, in which the mass ratio between the total mass of the polyoxyethylene alkyl ether and the polyoxyethylene alkyl ether acetic acid and the mass of water in the liquid phase ((polyoxyethylene alkyl ether+polyoxyethylene alkyl ether acetic acid)/water) is 60/40 to 95/5.