PURPOSE: To improve reliability by forming auxiliary air-actuation chambers in a vacuum brake power booster into two chambers and providing a hydraulic chamber interposing a seat-valve between a second movable wall to move in an opposite direction of the working direction of a master brake cylinder and a pressure fluid supply reserver.

CONSTITUTION: When a piston rod 11 moves to the left by operation of a brake pedal, a pressure difference in proportion to pedal power works on a movable wall 7 and power is polyploidized, a piston of a master key cylinder 2 is actuated through a push rod 62 and an intermediate piston 102, and hydraulic pressure is accumulated in a pressure chamber. This hydraulic pressure is supplied to each brake. When locking is caused during brake working, a second movable wall 64 moves to the right by an escape piston 121 (only a part is illustrated), and a second working chamber 45 is communicated to the atmospheric air. Thereafter, a compensating valve and a holding valve which are not shown in the drawing work are operated. In this constitution, it is possible to improve reliability.