PURPOSE: To save power consumption by using a reverse-rotation motor wherein an inner rotator and an outer rotator are reversely rotated as power of a grinder, and to make reaction in starting to a low level and the gear ratio for reducing speed small, so that the grinder may rotate quietly.

CONSTITUTION: An inner rotator 1 transmits power to a main bevel wheel 3 through an inner bevel wheel 2, while an outer rotator 4 also transmits power similarly to the main bevel wheel 3 through an outer bevel wheel 5, moreover these rotators are surrounded by a casing 6. The main bevel wheel 3, rotators a grinding wheel 7 through its shaft.