PURPOSE: To remove an auxiliary yarn as completely and rapidly as possible by delaying the cutting process in terms of time to a degree where a relatively long defective yarn is detected and at least the auxiliary yarn is not cut for each detection in a case of the defective yarn attributable to a piecing process.

CONSTITUTION: When the length of a knot is larger than or equal to the maximum value Lmax, the generated knot is detected as defective yarns 22, 22' attributable to the piecing using an auxiliary yarn. This means that the rear edge of the measured voltage coincident with an end part of the defective yarns 22 22' is generated at the time Tmax in terms of the measured voltage UM to be transmitted from a measurement sensor 14. The transmission of the cut commands TB, TB' to a yarn cutting device 16 is delay in terms of time. This prevents the auxiliary yarn from being cut. This means that the cut commands TB, TB' are generated for the first time after the yarn is cut attributable to the piecing at a winding part 12.