A treatment agent for polyolefin-based synthetic fibers according to the present invention is characterized by comprising a polyoxyalkylene derivative as mentioned below and a linear hydrocarbon compound as mentioned below. The polyoxyalkylene derivative is a compound in which at least one oxide selected from ethylene oxide and propylene oxide is added at a ratio of 5 to 100 moles relative to 1 mole of a monovalent aliphatic alcohol having 24 to 60 carbon atoms. The linear hydrocarbon compound is a linear hydrocarbon compound having 10 to 100 carbon atoms. A polyolefin-based synthetic fiber according to the present invention is characterized by having, attached thereto, the treatment agent for polyolefin-based synthetic fibers.