Synthetic fiber processing agent, and synthetic fiber

A synthetic fiber treatment agent comprising a smoothing agent and a nonionic surfactant, wherein

• the smoothing agent contains a condensed hydroxy fatty acid formed by condensation only from a hydroxy fatty acid having a hydroxy group and a carboxy group in the molecule,
• the smoothing agent further contains a modified silicone that is amino-modified silicone,
• taking the sum of the content ratios of the smoothing agent and the nonionic surfactant in the synthetic fiber treatment agent as 100% by mass, the content ratio of the condensed hydroxy fatty acid in the synthetic fiber treatment agent is 0.1% to 15% by mass, and
• the condensed hydroxy fatty acid is a castor oil fatty acid condensate or a hydrogenated castor oil fatty acid condensate.