A detector 22 detects action forces acting on a wheel 5 provided to a vehicle. An estimator 21a estimates the present value of a vehicle mass m on the basis of an action force acting in a component force direction out of the action forces, and an acceleration of the vehicle in the component force direction. Specifically, the estimator 21a estimates the present value of the vehicle mass m on the basis of an acceleration ax in the longitudinal direction and a longitudinal force Fx or on the basis of an acceleration ay in the lateral direction and a lateral force Fy. The estimator estimates the present value of a center-of-gravity height of the vehicle on the basis of the detected variation amount of the vertical force and the detected longitudinal force.