To make difference of pressure of gas in a compression chamber in an upper side of a piston and pressure of gas in a crankcase as small as possible in a booster type gas compressor having the piston reciprocated in a cylinder connected to a crankcase and compressing and delivering compressed gas introduced to the compression chamber in the upper side of the piston.

Part of compressed gas in a compressed gas transfer path 34 before an intake valve 23 is introduced in the crankcase 29 under a sealed condition to maintain pressure in the crankcase 29 at ambient pressure or higher by connecting the pressurized gas transfer path 34 and the crankcase 29 under the sealed condition with a branched path.