PURPOSE: To increase the reliability of operation and reduce a cost by transmitting a boost force acting on a first movable wall directly to a control housing in which a valve piston is stored in a subject booster in which two movable walls are provided in a housing.

CONSTITUTION: An inside of a housing is divided into front and rear chambers 20 and 21 by a partition, and the front and rear chambers 20 and 21 are divided into negative pressure chambers 32 and 33 in which a pressure is held at a constant and power chambers 24 and 25 in which a pressure is variable by movable walls 14 and 15, respectively. A control housing 4 including a guide part 44 is installed slidably on the rear side half body 2 of the housing, a control rod comprising a piston rod 7 and a valve piston 6 are fitted into the inside of the housing, and a poppet valve 8 operated by the valve piston 6 is stored. A push rod 10 connected to a master cylinder through a reaction disk 9 is connected to the valve piston 6. Then an axial extension part 22 is installed in a control housing 4, and the opening edge of the partition 41 is connected to its tip end.