PURPOSE: To aim at miniaturization by installing an electric motor having a hollow output shaft in the same axis around a spindle, supporting the output shaft on a bracket of an attachment body free of rotation, and connecting the output shaft directly to the spindle via a speed change mechanism.

CONSTITUTION: A spindle is rotatably supported on an attachment body 1 via a bearing 2. The attachment body 1 is supported is rotatably on a bracket 5 and a tuning shaft 7 on top of this bracket 5 is supported by a body 9 free of rotation. In an interval between the attachment body 1 and the spindle 3, there is provided with an electric motor 11 having a hollow output shaft 12 of the same axis with the spindle 3, and rotation of this output shaft 12 is transmitted to the spindle 13 via a speed change mechanism consisting of hears 16, 17, 18 and 20. Accordingly, a driving device of the spindle and the speed change mechanism are installed in the inner part of the attachment body 1, thus miniaturization is made possible and, what is more, kinematic performance is improved.