When a brake pedal is operated by the vehicle driver, and detection signals from hydraulic pressure sensors and a brake switch, for detecting pressure of the brake pedal, are supplied to an ECU, the ECU performs a determination according to the detection signals as to whether or not the hydraulic pressure PM is a maximum pressure gradient d(PM)/dtmax. When the determination result of the braking condition is an emergency braking condition, an intake valve is opened. Thereafter, atmosphere is forcibly supplied to atmospheric chambers of a booster, to assist the braking force.