To more precisely perform the inspection of a yarn defective part by comparing the actual tension measured by a yarn tension measuring member with a threshold in a yarn traveling state to detect the yarn defective part.

When the traveling signal Vf of a spinning yarn is ON, whether it is before a prescribed time or not is judged (F1). In case of NO, calculation of no-load tension is waited until a prescribed time passed. After the spinning yarn is engaged with the yarn guide of the detecting arm of a tension measuring member, the tension after the lapse of the prescribed time is measured to calculate the actual load tension reference value (F3). The threshold for judging a yarn defective part or not on the basis of the difference between the no-load tension reference value and the actual load tension reference value is determined on the basis of the no-load tension reference value and the actual load tension reference value (F4). It is then judged whether the actual tension measured by the tension measuring member is lower than the threshold or not (F5).