To calculate a stability factor of a vehicle according to the state of the vehicle in connection with the technique learning the stability factor of the vehicle.

The state of the vehicle during traveling is detected and it is determined (step B90) whether the present traveling state of the vehicle is in a stable turning state or not based on the detection information. If it is determined that the present traveling state of the vehicle is in the stable turning state, the stability factor of the vehicle is calculated (step B140) by the predetermined computing equation based on the vehicle speed, the steering angle, and the yaw rate detected during traveling. The stability factor in the present state of the vehicle is obtained by setting the calculated value to be the learning value (step B190) of the stability factor of the vehicle.