To prevent malfunction of fluff detection caused by running yarn bouncing phenomenon or malfunction of end breakage detection due to dust.

Two places, in the front and rear of running yarn 11 or two places near the front and rear of the running yarn 11 are irradiated by dividing the laser beam of a single laser beam source 13 into a front side laser beam and a rear side laser beam by a blocking member and the beams are received by a pair of photodiodes 14, 15. Malfunction of bouncing phenomenon is prevented by inputting output signals from both beam receiving elements 14, 15 into a differential amplifier and identifying the fluff detection on the basis of the difference. Malfunction caused by dust is prevented by detecting end breakage by identifying whether output signals from both beam receiving elements 14, 15 make changes within a specific time or not by an AND circuit 38.