PURPOSE: To enable speedy calculation, to reduce an error, and to improve the reliability by calculating driving torque to be transmitted to a slip wheel and the load on the slip wheel, and calculating the coefficient of maximum friction between a road surface and a tire from those torque and load, and the diameter of the tire.

CONSTITUTION: The front and rear wheel angular acceleration calculation part of a control unit 30 calculates the angular acceleration values βf and βr of the front and rear wheels and the front and rear load calculation part 32 calculates the loads Wf and Wr on the front and rear wheels. The front and rear wheel torque calculation part 34 calculates the torque values Tf and Tr of the front and rear wheels from engine torque Te and a torque distribution ratio. The slip detection part 35 compares vehicle body acceleration G with the Bf and Br to detect whether or not a slip is made. In this case, when a front wheel slip is detected from Gβf.r (tire diameter) for the torque distribution of front wheel unbalanced loading, the road surface μmax detection part 36 calculates maximum road surface friction coefficient μmax=Tf/r.Wf. Then torque is distributed to the rear wheels a specific quantity more so that transfer torque Tf≤μmax.r.Wf holds together with the transfer pressure of a transfer clutch 11, thereby securing preventing the front wheel slip.