PURPOSE: To have efficient takeout of high pressure air by placing the piping pressure in communication with the crankcase of a compressor.

CONSTITUTION: Pressurized air, being supplied to a suction chamber 25 and cylinder chamber 9, is also fed into a crankcase 3 via a divergent pipe 6. In the initial period of pressurization by a piston 7 sinking, the pressures above and below the piston 7 are balanced, and a weak force raises the piston 7. High pressure air produced by rise of the piston 7 is sent through a discharge valve 23 to be stored in a tank 31. Pressurization by an air pressure boosting device 1 will generate high pressure air easily. Further the vertical motion of piston 7 is smoothed by feeding the crankcase with this pressurized air, which contributes to holding the crank shaft 10 well balanced to lead to prevention of ripple generation in the air pressure and the machine.