PURPOSE: To immediately increase the number of normal rotations of a rotary cutter and to enable the surfacing of high accuracy by setting the maximum allowable max. value of a cutting load resistance and providing a control part for controlling a cutting feed motor by its deceleration at the time when an actual cutting load resistance becomes larger than this value.

CONSTITUTION: At the main motor 14 side for driving a rotary cutter a detector 40 for detecting the cutting load resistance thereof is provided. The output of this detector is converted into a voltage signal by a convertion circuit 41, and compared with the set value set by a load setting circuit 42 by a comparison circuit 45. As a result of this comparison, in case of the actual load resistance exceeding the max. allowable value(set value) of the cutting load resistance, the cutting feed motor 19(20) for driving a cutting head is controlled by its deleration by a control part 35, an optimum cutting is made, the rotary cutter is returned to a normal rotation and surfacing is made of high accuracy.