PURPOSE: To enable it to do a smooth disengagement even in a state that torque has worked, by installing a differential gear mechanism with a torque amplifying function and an externally operable claw clutch for connecting an interval between optional differential gears, respectively.

CONSTITUTION: When a differential of a mechanism 75 is locked by a clutch 85, its driving force being inputted via a rear axle 23 and a clutch 91 is transmitted to another rear axle 21 as it is. When an operating ring 97 is operated to be moved to the right from this state, a disc spring 89 is bent, a clutch member 81 is slid to the right, and the clutch 85 is thus opened. Since force to be imposed on a sun gear 63 is decreased to amount 20% of man power, sliding resistance in a meshing part of the clutch 85 at this time becomes lessened as little as for that, therefore engagement is smoothly released. When the clutch 85 is opened, the sun gear 63 runs idle by rotation of a planetary gear 53 and the driving force is not transmitted to the rear axle 21 at all, thus it comes to a free rotational state.