PURPOSE: To reduce the dimension of a vacuum booster for brake device for a vehicle by operating a piston with air supplied into a power cylinder part, accompanied with the brake operation, and by supplying the working pressure into the brake with the operation of a working pressure feeding cylinder.

CONSTITUTION: The primary pressure is supplied from a master cylinder to one edge of the rear piston 56 in a relay valve part 40 through a working pressure feeding cylinder part 44 and a working oil passage 60 by the operation of a brake pedal. At the same time, the primary pressure is supplied also to one edge of a front piston 54. Accordingly, both pistons 56 and 54 shift in the upper left part on the figure by the primary pressure, a valve body 46b is then closed by shifting a diaphragm 48 leftward, and air is taken in by opening a valve body 46a. The air is introduced into the first pressure chamber 36 of a power cylinder part 32. Since the second pressure chamber 38 is kept at a low pressure, a power piston part 34 and a pushing rod 63 are moved by pressure, and the first plunger 64 is moved by pressure, and the working oil set between the second plunger 66 is pushing-supplied into a rear brake.