PURPOSE: To compress air in two stages during one full rotation of a piston by disposing an annular rotary drive wheel between an annular rotation outer wheel and an annular rotation inner wheel, arranging a piston to the annular rotary drive wheel in a manner to be protrusive and retractable, and forming primary and secondary pressure chambers.

CONSTITUTION: An annular rotary drive wheel 12 is disposed in a casing 3 between annular rotation outer and inner wheels 11 and 13, which have the same rotary axis as each other. At least two pistons 10A and 10B are producibly and retractably disposed in a direction extending vertically to the rotation axis of the annular rotary drive wheel 12. Further, the head parts of the piston 10A and 10B are nipped between the annular rotary outer and inner wheels 11 and 13. By displacing the rotary axis of the annular rotary drive wheel 12 from the rotary axis of the annular rotary inner wheel 13, primary and secondary pressure chambers 30 and 31 are respectively formed. This constitution compresses gas in two stages during one full rotation of the pistons 10A and 10B.