PURPOSE: To prevent vibrations or noises from being generated, by arranging a driving mechanism for driving a collet chuck as well as a motor for driving a main shaft coaxially with the main shaft thereby arranging the main shaft and the motor in parallel with each other.

CONSTITUTION: The collet chuck mechanism 14 consists of a collet chuck 11, a plate spring 13, an push rod 29. An output shaft 44a of the motor 44 secured from the opposite side of main shaft table to a box 33 by a bolt 45 is extended into the box 33 coaxially with the rod 29. A connecting member 44 is connected to an end of the main shaft 2 by a key 16, a distance plate 18 and a cap nut 19, and the other end is connected to the output shaft 44a of the motor 44 by a key 47. A tool holder 5 is fastened to the main shaft 2, and after the piston 40 is stopped and the motor is driven, the main shaft 2 connected to the output shaft 44a of the motor 44 is rotated via the connecting member 46.