To reduce difference pressure applied to a bellows so as to prolong the life of the bellows and miniaturize the device by forming a compression chamber and an auxiliary compression chamber to be partitioned off with bellows forming compression chambers by means of a piston reciprocating in a cylinder according to rotation of a crank.

One and the other chambers partitioned off with a piston 1 are airtightly sealed from each other by a sealing means 2 inserted between the piston 1 and the cylinder 10. The piston 1 is reciprocated in the cylinder 10 by rotating a crank 11, and hence the volumes of a compression chamber 5 and an auxiliary compression chamber 6 divided with a bellows 3 inserted between the one end of the cylinder 10 and the piston 1 are changed. Gas in the compression chamber is discharged from a discharge port 12 formed on the one end 101 of the cylinder 10. Pressure in the auxiliary compression chamber 6 is changed according to movement of the piston 1 due to rotation of the crank 11, and pressure difference of both chambers applied to the bellows 3 is reduced.