To keep a sawing part in a linear cutting state to a work by applying resistance of specified torque to the rotating force of a driven regulating wheel.

A second gear 13 is given specified friction resistance by a friction discs 15, 15 and the resistance applies load to an idle pulley 3 with constant torque via a series of transmission mechanisms. When a work W is pressed on the sawing part 36 of a band saw 4 and is cut in this state, the sawing part 36 has large tension in a driving pulley 2 side and small tension in the idle pulley 3 side with the boundary of the work W, but load with constant torque is always applied on the idle pulley 3, whereby the idle pulley 3 side of the sawing part 36 is not deflected and remains tensioned.