PURPOSE: To accurately set wire tension, by interposing the first differential gear mechanism to exist in a rotary transmitting route, which reaches at least one of the tension rollers from a driving motor, and connecting a control shaft of the mechanism to a reversible tension control motor.

CONSTITUTION: A wire 2, fed from a reel 3 in accordance with a position of a dancer roller 5, is wound to a reel 4 in accordance with a position of a dancer roller 6 via a tension roller 7, sawing head rollers 10W12 and a tension roller 8 driven by a driving motor 9. The tension of the wire 2 between the rollers 7, 11 and between the rollers 12, 8 is detected respectively by sensors 52, 53, and in accordance with a signal of these sensors, the first and second differential gear mechanisms 21, 22 control rotation of their reversible control motors 33, 34 changing a speed to the tension roller 7 via gear trains 17, 18 while to the sawing head rollers 10W12 via a timing belt 25 from the driving motor 9 and controlling the tension to a fixed value. In this way, accurate control of the tension can be performed by a quick response speed.