PURPOSE: To cut off a hollow work continuously formed at high speed and reduce heat generation at cutting.

CONSTITUTION: It is constituted out of a mechanism part consisting of a sun gear 2, a ring gear 3, and planet gears 4, and cutting tools 5 fitted with the planet gears 4. By controlling rotating speed of the sun gear 2 and the ring gear 3, at first autorotational movement is given to the planet gears 4 to approach the cutting tools 5 to a work 6 passing through in the sun gear 2, next to this, while giving the planet gears 4 revolutional movement, autorotational movement is given to the planet gears 4 to cut off the work 6 with the cutting tools 5.