To provide a loosened yarn-tightening device incorporating an electromagnetic tenser, achieving stable rotational resistance with respect to a loosened yarn-tightening roller of a thread guard member and forming packages with stable quality.

The loosened yarn-tightening device incorporating the electromagnetic tenser is provided with a rotating drive source 25, the loosened yarn-tightening roller 21 rotated and driven by the rotating drive source 25 and the thread guard member 22 concentrically and relatively rotatably mounted to the loosened yarn-tightening roller 21. A magnetic field formation means by an electromagnet is provided in the loosened yarn-tightening roller 21 such that a magnetic field is applied to the thread guard member 22. Due to the magnetic field generated by the magnetic field formation means, rotating torque is generated between the loosened yarn-tightening roller 21 and the thread guard member 22, and by input control to the electromagnet, the rotating torque is controlled.