A treatment agent for a polyolefin-based synthetic fiber capable of imparting a high degree of hydrophilicity to an obtained nonwoven fabric while maintaining good antistatic properties and card passing property and capable of weakly acidifying the nonwoven fabric, an aqueous solution of such a treating agent, a polyolefin- A method of treating synthetic fibers, polyolefin-based synthetic fibers to which such a treating agent is adhered, and thermal-bonded nonwoven fabrics of such polyolefin-based synthetic fibers. As the treatment agent for polyolefin-based synthetic fibers, 0.3% by mass to 18% by mass of a specific organic acid, 10% by mass to 50% by mass of alkyl phosphoric acid ester salt B having 4 to 11 carbon atoms, and alkyl phosphoric acid ester salt C having 10 to 22 carbon atoms By mass to 50% by mass and a specific polyoxyalkylene derivative in a proportion of 25% by mass to 65% by mass (total of 100% by mass), and the pH of the specific aqueous solution is 3 or more and less than 7 .
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