PURPOSE: To boost brake force even upon actuation of an auxiliary brake pedal, by making it possible to actuate a piston of a hydraulic cylinder actuated by a main brake pedal, for driving a booster, even by means of hydraulic pressure from an auxiliary brake master cylinder.

CONSTITUTION: A hydraulic cylinder 10 is interposed between a pushrod 3 coupled to a main brake pedal and a brake booster 6, and is inserted therein with first and second pistons 11, 12, the first piston 11 being engaged with the pushrod 13 and the second piston 12 being engaged with a booster operating rod 14. The second piston 12 is formed with a protruding part 13 engaged with the first piston 11, and a chamber 18 is defined between both pistons 11, 12. An auxiliary brake master cylinder 7 is connected to this chamber 18 through an inlet port 17.