To provide a cut thread treatment device for synthetic fiber and a method therefor capable of detecting the cut of a single thread just below a nozzle plate early, restoring only a thread bar in which single thread is cut automatically without interrupting the production of a whole thread bar, improving product yield, and reducing loss in operation when a multifilament is produced by a multithread bar high speed spinning method using a horizontal spraying device.

A cut thread treatment device for synthetic fiber is provided in which a light source 7 is provided on the downstream side of a thread bar on a side opposing to a cooling air spraying part of a spraying device across the thread bar, a cut thread detector 11 having a light receive body is provided on the upstream side thereof, and suction nozzle 10 sucking a cut thread in the interlocking relationship with a signal of the cut thread detector 11 so as to travel is provided in a spinning device which melts and spins fiber forming thermoplastic polymer, cooling it by a horizontal spraying device 2, then giving an oil solution to it, and collects thread bars by a collection guide 4.