PURPOSE: To switch transmitted torque by sliding of a slider, by mounting a magnetic member on a confronting face of a gear and a slider.

CONSTITUTION: An output gear 1 is pivotally mounted on a center shaft and a hexagon body portion is integrally fixed to a boss portion 1a. An input gear 2 is mounted on the boss portion 1a of the output gear 1 through an input gear in such a manner that it cannot slide in an axial direction and is relatively rotatable with respect to the output gear 1. A slider 5 coaxially having a hole 5a is mounted on the hexagon body portion 8. The slider is mounted in such a manner that it can slide in the axial direction and cannot effect a relative rotation in a rotational direction. Magnets having different poles are radially disposed on the input gear 2. A magnetic member 4 is mounted on an upper surface of the slider 5 and transmitted torque can be switched by the sliding of the slider 5.