A weight-related physical quantity estimating system and a control device according to the present invention are applied to a vehicle including a braking force changing device for changing the relationship between the braking force of front wheels and the braking force of rear wheels in accordance with the position of a displacement member which moves in accordance with the supporting load of the rear wheels. The weight-related physical quantity estimating system estimates the supporting load of the rear wheels on the basis of the braking forces of the front wheels and the rear wheels, and estimates at least one of the longitudinal position of the gravity center of the vehicle, the supporting load of the front wheels and the total weight of the vehicle, as a physical quantity relating to the weight of the vehicle. The control device modifies a value which is used for controlling the vehicle and is influenced by the physical quantities relating to the vehicle weight, on the basis of the braking forces of the front wheels and the rear wheels.