The present invention provides a polyester-based synthetic fiber treatment agent that simultaneously satisfies the requirements of emulsion stability, heat resistance, antistatic property of the carding step in the weaving process and scum resistance of the drawing step, and does not cause degradation of the fiber strength, a treatment method of the polyester-based synthetic fiber using the treatment agent, and a polyester-based synthetic fiber obtained by the treatment method. As the treatment agent for the polyester-based synthetic fibers, a specific alkyl phosphate is contained in a proportion of 40 to 80% by mass, a specific surfactant is contained in a proportion of 10 to 50% by mass, and a monovalent aliphatic alcohol is contained at a ratio of 0.01 to 10% by mass, wherein the monovalent aliphatic alcohol has an alkyl group having 12 to 22 carbon atoms in the molecule; said components add up to 100 mass%.