PURPOSE: To detect a yarn defect even when yarn is placed in and run by impressing previously a voltage which cancels a signal voltage component of the output signal of a yarn defect detecting circuit corresponding the mean thickness of the yarn, and cutting off the impression of the voltage a specific time after the yarn is placed in.

CONSTITUTION: When there is no yarn in a measuring head 21, the output signal of a measuring circuit 2 is zero and an electronic switch 6 is closed; and the voltage VR of a power source 5 is impressed to the input side of an inverting amplifier 4, which outputs a minus signal V0. Once yarn is placed in the head 21, the switch 6 is opened and a capacitor 7 begins to be charged. The capacitor 7 while charges are conductive and the voltage VR is still impressed, so that a signal obtained by removing the voltage VR corresponding to the mean thickness of the yarn from the signal Vy of the amplifier 3 is inputted to the amplifier 4. When the capacitor 7 is charged completely and turns off, the impression of the voltage VR is cut off and the input signal contains only variation in the weight of the yarn, so that a defect of yarn regardless of its thickness even when it is placed in the head and run.