To provide a synthetic fiber for a small-diameter electric wire cord, which hardly causes breaking due to bending fatigue and the like and an interrupt of electric conduction, and has stable performance.

The synthetic fiber for a small-diameter electric wire cord comprises: a synthetic fiber made of a multifilament; and a treatment agent containing a vapor phase corrosion inhibitor, which is deposited on a surface of the synthetic fiber. In addition, the synthetic fiber is preferably an aromatic polyamide fiber. The treatment agent preferably contains a monobasic acid ester having a molecular weight of 250-600, is an aqueous treatment agent, contains an ether compound prepared by adding 3-9 moles of propylene oxide and/or ethylene oxide to a 8-15C alcohol, and contains an anionic activator. In addition, the vapor phase corrosion inhibitor is preferably a compound having a triazole ring and a benzoate-based compound.