PURPOSE: To develop a tension controlling device of a coil winding machine that can absorb slackening of wire due to inertia at the time of winding and is excellent in responsiveness by utilizing a specified hysteresis brake system on electric wire at the time of winding of a coil winding machine.

CONSTITUTION: When winding an electric wire 22 by a coil winding machine 19 on a bobbin 21 through a guide roll 16, a hysteresis brake 2, a guide roll 15, a tension roll 9 and a guide roll 18, if the wire 22 stops, slackening is generated in the wire due to inertia of the hysteresis brake 2, and the back tension roll 9 goes down and slackening is absorbed mechanically. At this time, the turning angle of the back tension arm 7 is detected by a potentiometer 1 using a turning angle of a turning shaft 3. When slackening of the wire 22 becomes larger, the voltage of the hysteresis brake is controlled to increase the braking force of the hysteresis brake, and such inertia is absorbed.