PURPOSE: To properly estimate yaw rate in the relation with the traveling state of a vehicle, from the revolution speed difference between the left and right wheels.

CONSTITUTION: It is judged if a vehicle is on a bad road or good road, from the vehicle acceleration speed (S310), and when the vehicle is on the good road, the average quantity N in averaging the yaw rate at this time by the final yaw rate in the preceding time is set as the average quantity N1 for good road (S315). While, when the vehicle is on the bad road, the average quantity N is the average quantity N2 for bad road and has the value lager than N1 (S320), and if the absolute value of the steering angle differential DΘ is larger than the threshold value KDΘ, the average quantity N is the average quantity N1 for good road (S335). Since the average quantity N is varied according to the traveling state of the vehicle, both of the road surface noise resistance on the bad road and the responsiveness on the good road can be achieved.