PURPOSE: To reduce the number of component parts and power loss, and eliminate need of a shaft sealing device by compressing air or gas by circulating two pistons in circular cylinder.

CONSTITUTION: While one piston 21 is stopped at a first position 4 by a first electromagnet 41, the other piston 22 is guided by a first magnet 31 in the direction of an arrow 3 through a compression and suction stroke to a second position 15. At this time, air or gas in a cylinder 10 in the upstream of the other piston 22 is compressed to be discharged from a discharge port 13 through a discharge valve 16. Simultaneously, air or gas is sucked from a suction port 12 into a space formed in the cylinder 10 in the downstream of the other piston 22. A second magnet 32 advance following the first magnet 31. When the other piston 22 reaches the second position 15, a second electromagnet 42 is energized, so the other piston 22 is detached from the first magnet 31 to be stopped at the second position 15, thereby one stroke of the compression and suction stroke is completed.