PURPOSE: To enable starting on an uphill road and the like safely by automatically performing damping by operating a brake device when a car stops and an accelerator pedal is released and by sealing damping liquid pressure with a brake operation system.

CONSTITUTION: When a car stops on the way of an uphill road and stepping operation of an accelerator pedal is released, each of solenoid 50 and 60 of an electromagnetic switch valve 5 and an electromagnetic switching valve 6 is magnetized by a control unit C which receives each output signal of a car speed detecting element S1 and an accelerator opening detecting element S2. A control valve in a brake booster 1 is opened by an actuator 4 whose driving source is an intake negative pressure, and the atmosphere is introduced into a transforming pressure chamber P2. Thus, pressure difference is generated between the transforming pressure chamber P2 and a fixed pressure chamber P1, and a master cylinder 20 is operated through a booster piston by this pressure difference, and brake 22 and 25 are operated. Further, simultaneously, an electromagnetic valve 70 is opened and brake liquid pressure is sealed, thereby damping condition is maintained.