A power distribution control device 42 calculates a low-.mu. information magnitude, associated with the friction coefficient of the road surface, based on the slip rate and the vehicle acceleration operation magnitude. If the low-.mu. information magnitude exceeds an addition evaluation threshold, the power distribution control device 42 adds the low-.mu. information magnitude. If the low-.mu. information magnitude does not exceed the addition evaluation threshold, the power distribution control device 42 subtracts a constant K from a counter. The power distribution control device 42 makes evaluates a road to have a low-.mu. when the counter exceeds a low-.mu. evaluation threshold.