[PROBLEMS] Sufficiently compatible with recent high-speed spinning processes, web uniformity in high-speed card process, roller wrapping resistance in high-speed ring drawing process, resistance to scum deposition, coil foam, high-speed ring Polyester synthetic fiber spinning treatment that simultaneously satisfies the scum deposition resistance in the spinning process and the scum deposition resistance in the high-speed rotor open-end process, a method for treating a polyester synthetic fiber using such a treatment agent, and such a treatment method To provide a polyester-based synthetic fiber. As a processing agent for spinning a polyester-based synthetic fiber, 10 to 90% by mass of a specific alkyl phosphate alkali metal salt having an alkyl group having 12 to 22 carbon atoms in the molecule and 5 a specific surfactant. -80 mass% and containing a specific alkyl phosphate ester amine salt having a C12-22 alkyl group in the molecule in a proportion of 0.1-45 mass% (total 100 mass%) It was. [Selection figure] None
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