PURPOSE: To take a new lease of life of a rotary piston which houses a cylindrical piston in a circular body so as to freely rotate eccentrically by reducing an abrasion of a seal by providing a pair of eccentric member at the piston and controlling an orbital movement of the piston.

CONSTITUTION: A cylindrical piston 37 is housed in a chamber 40 which consists of a circular body 24 and edge plates 27, 28 which are attached to the both sides of the circular body 24 supported by an axis 42 provided with an eccentric boss 46 so as to rotate freely. This piston 37 is provided with a pair of eccentric members 104, 106 related to operations to control the orbital movement. Axes 113, 110 are provided on the both ends of each eccentric members 104, 106 and supported with axis at edge boards 27, 28 through a bearing. Seal assemblies 126-130 which have a pair of vanes 131, 132 at even intervals toward a circumference respectively around the piston 37 and the both ends of a rail 153 provided at bottoms of each vane 131, 132 are guided to an arc groove hole 159 so as to slide freely.