Abstract: In the method, the number of defective portions of the yarn, occurring at periodic spacings within a predetermined time period, is detected to provide a detected value. This detected value is compared with a predetermined desired value of the number of defective portions within such predetermined time period. If the detected value exceeds the desired value, an output signal is provided or released to a stop device, control device, or the like of the spinning machine. The device comprises a thread monitor connected to a pulser which produces pulses responsive to variations in the yarn thickness. The output of the pulser is supplied to a thick portion monitor which detects long pulses produced responsive to relatively long defects in the yarn, and the thick portion monitor is connected to a time switch which, in turn, is connected to a signalling device, stop device, or control device for the spinning machine.