PURPOSE: To provide a fiber-treating agent imparting flexibility to the fibers and especially preventing the yellowing of the fibers and the rusting of wires, etc., in a spinning process by compounding a specific alkyl sulfonate salt, a phosphate salt of a specified addition compound, polyphosphoric acid and a specific amine salt in a specified ratio.

CONSTITUTION: (A) A 12-15C alkyl sulfonate alkali metal salt (e.g. lauryl sulfonate salt), (B) a phosphate alkali metal salt of a compound prepared by adding 0-20 moles of an alkylene oxide to a 12-18C higher alcohol (preferably a phosphate salt of a compound prepared by adding 2-4 moles of ethylene oxide to laurylalcohol, etc.), (C) polyphosphoric acid and (D) 12-18C alkylphosphate amine salt are compounded in a weight ratio of A/B/C=63-88/5-30/2-3 and (A+B+C)/D=80-95/20-5 and adjusted to a pH of 4-5 to obtain a fiber-treating agent which imparts good smoothness and flexibility to fiber products, prevents the yellowing of the fibers and suppresses the rusting of wires, etc., of a carding machine in a spinning process.