Abstract: The invention relates to a brake system including a brake booster. A pneumatic pressure operated brake booster VBB or a liquid pressure operated brake booster includes a valve mechanism which is urged by a force of depression applied to a brake pedal BP to switch a flow path to cause the brake booster to develop an output which depends on the magnitude of the force of depression. A solenoid SOL urges the valve mechanism in the same direction as or in the opposite direction from the force of depression. A controller ECU is responsive to a braking effort increase/decrease demand signal to increase or decrease the urging force which is applied by the solenoid to the valve mechanism, thus increasing or decreasing the output from the brake booster. An output from the brake booster can be freely controlled independently from the force of depression applied to the brake pedal in response to a, braking effort increase/decrease demand.