PURPOSE: To control the cutting load constant even if the contact area between a shape steel and a saw is changed by feeding back the rotating speed change of a prime mover rotatively driving the saw to the operating speed of an actuator to keep the rotating speed of the prime mover constant.

CONSTITUTION: A shape steel 6 is cut by a saw 2 while a prime mover 1 is driven at a preset speed and an actuator 5 is operated at a fixed speed. When the area of the shape steel 6 cut by the saw 2 is changed as it is cut, this change becomes the load fluctuation for the saw 2, and the rotating speed of the prime mover 1 is changed. An error of the rotating speed of the prime mover 1 from a reference value is determined, and the corrected cutting speed is calculated from the determined error signal VM. The actuator 5 is drivingly adjusted by a servo valve 17 based on this calculated value, and the prime mover 1 is controlled so that its rotating speed is kept constant. That is, the rotating speed change of the prime mover 1 is fed back to the operating speed of the actuator 5, the rotating speed of the prime mover 1 is controlled, and the cutting load of the saw 2 is kept constant.