PURPOSE: To improve durability by moving a propeller shaft in the axial direction to connect a clutch through relative rotation between a casing and an axial flow propeller, and increasing transmitting torque corresponding to increase of the rotating speed difference along a smooth progressive curve.

CONSTITUTION: When rotating speed difference is generated between front and rear driving axles 83, 86, rotating speed difference is generated between a first and second rotating shafts 11, 12, and as one side rotating shaft rotates by the rotating speed difference relatively, an axial flow propeller 6 receives a thrust F in the axial direction by stirring oil. When this thrust F moves a propeller shaft 4 and a clutch plate 32 to the left against a compression coil spring 5, the clutch plate 32 is pressed against the friction face 31a of a friction clutch and connected. Consequently, the shaft 4 and a casing 2 can be rotating in one body and both shafts 11, 12 are rotated together with. When the rotating speed difference between both shafts 11, 12 becomes small, the spring 5 moves the shaft 4 and the plate 32 to the right against the thrust F, the plate 32 is separated from the friction plate 31a, and the rotating speed difference between both shafts 11, 12 becomes in allowable condition.