PURPOSE: To enable a ring to be cut with a small force, to enable a driving part for driving support rods to be made smaller, and to enable a cutter to be miniaturized.

CONSTITUTION: A drive shaft portion 5 comprises cutting blades 6 each provided with a pair of support rods 7a, 7b by which the cutting blade 6 is supported in such a way as to freely rock in the direction in which it moves in and out, and a switch means V is provided for switching the operating condition,of both support rods 7a, 7b between a first state in which the ends of both support rods 7a, 7b move toward a ring T at almost the same speed and a second state in which the end of only one support rod 7b moves toward the ring T, and a pressure sensor for detecting cutting pressure is provided. A controller 9 is provided for switching the switch means V to the first state if the detection result of the pressure sensor shows that the cutting pressure is smaller than a set value and for switching the switch means V to the second means if the cutting pressure is at or greater than the set value.