To improve the monitoring of a traveling spun yarn in a measurement area by correcting measurement errors in relation to the present position of the yarn.

In an open end spinning machine, a formed yarn 3 is extracted by a extraction roller pair 5 via an extraction tube piece 4 from a spinning box 1 and is wound to form a traverse package 10. A sensor device 6 and a sensor device 18 for monitoring the traveling yarn 3 are connected to a data processing device 15 receiving and processing the detected signals. Measurement values detected by the sensor devices 6, 18 are compared with each other each time, the measurement errors caused by a rocking process are detected and compared at each position of the yarn 3 in the measurement gap of the sensor device 6. Since the measurement errors are corrected at the present time, the measurement errors can be reproducibly corrected, thus the quality of the spun yarn can be guaranteed.