PURPOSE: To make it possible to obtain a predetermined boost ratio without making a vacuum boosting device large, by utilizing an air ejector to supply high vacuum to the vacuum boosting device.

CONSTITUTION: An air ejector 10 having an air-intake port communicated with the atmosphere is connected to a vacuum extracting hole 8 arranged downstream of a throttle valve 7 in an internal combustion engine E, and a vacuum chamber 3 in a vacuum boosting device S is connected to a pressure reducing chamber 15 in the air ejector 10. Further, a first check valve 23 is disposed in a bypass passage 22 which bypasses the air ejector 10 and which has a flow resistance lower than that of the air ejector 10, and a second check valve 24 is disposed between the bypass passage 22 and the vacuum chamber 3. Accordingly, even if the intake vacuum pressure in the internal combustion engine is low, high vacuum may be fed to the vacuum boosting device S, and also the engine intake vacuum may be immediately supplied to the boosting device until the vacuum pressure of the vacuum chamber reaches the intake vacuum pressure of the engine, thereby the accumulation of vacuum may be made soon.