PURPOSE: To prevent spinning of ending parts having thickness different from other parts, by detecting the thickness of unwound yarn from a bobbin, making a memory memorize its value, detecting the thickness of ending parts, comparing it with the memorized detected value.

CONSTITUTION: The thickness of the unwound yarn Y from the bobbin 8 is detected by the detector 16 of the unit 15, and its value is memorized in a memory. The thickness of the ending part connected is detected by the detector 16, and its value is compared with the memorized value. When the thickness of the ending part exceeds a tolerance value, the yarn is cut by the cutter 19. The thickness of the ending part can be compared with that of the yarn which is being spun.