PURPOSE: To detect twining yarn in early stage by arranging a pair of infrared detectors while separating by predetermined distance from the surface except the yarn contacting section of roller and facing each other then detecting the difference between the outputs.

CONSTITUTION: The roller 10 will rotate with constant speed to feed the yarns Y1, Y2. Infrared detectors 21, 22 for detecting the twining yarn are arranged while facing against the surface of said roller and an infrared detector 23 for correcting the variation of the surface condition of roller 10 is further provided. Upon twining of yarn Y1, the radiation energy from the surface of roller 10 will vary to lower the output from the infrared detector 21. The detection signal D1 is subtracted of correction signal C from the correction infrared detector 23 through subtractor 24 and increases as the twining amount will increase thus to provide a twining signal O1 from a comparator 26 upon exceeding over a setting signal S.