PURPOSE: To simplify the structure of a machining device and to reduce the size thereof by locating the drum part of a plurality of sets of rollers so that the rollers are brought into contact with the inner surface of a friction ring mounted in the central hole of a driven gear for revolving a roller and supporting a machining reaction force, exerted on a rotor, by means of the friction ring.

CONSTITUTION: The drum parts 4 of three or four sets of rollers are brought into contact with the inner surface of a friction ring 3 mounted in a central hole 2 of a driven gear 1 for revolving a roller. Two end shaft parts 7 of the roller are supported by respective bearings 8 in orientation wherein a rotary shaft 5 of the roller is twisted in the same direction around a machine central axis 6. The gearing 8 is held in each of driven gear pair 9 and 10 for revolving a roller located on both sides of the driven gear 1 to enable revolution of a roller pair. Based on the gear pairs 9 and 10, the position of the roller is regulatable in the direction of the roller rotary shaft 5 by means of a depressing mechanism 11, and the driven gear 1 and the gear pairs 9 and 10 are driven in a distributing manner by means of distribution gears 13-15 of the same drive shaft 12.