Provided are: a treatment agent for polyolefin-based synthetic fibers, which is capable of imparting a nonwoven fabric with high hydrophilicity, while maintaining good antistatic properties and card passing properties, and which is also capable of making the nonwoven fabric weakly acidic; an aqueous solution of this treatment agent; a treatment method for polyolefin-based synthetic fibers, which uses this aqueous solution; polyolefin-based synthetic fibers to which this treatment agent is adhered; and a thermally bonded nonwoven fabric which is formed from these polyolefin-based synthetic fibers.The present invention uses a treatment agent for polyolefin-based synthetic fibers, which contains 0.3-18% by mass of a specific organic acid, 10-50% by mass of an alkyl phosphate salt B having 4-11 carbon atoms, 10-50% by mass of an alkyl phosphate salt C having 12-22 carbon atoms and 25-65% by mass of a specific polyoxy alkylene derivative (with the total of the components being 100% by mass), and which provides a specific aqueous solution having a pH of 3 or more but less than 7.
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