PURPOSE: To make smooth the rotation of a rotor, by forming the inner peripheral surface of an outer casing into a deformed circle such as an oval, and by making round the outer peripheral surface of the rotor rotating therein.

CONSTITUTION: A rotor 3 rotates smoothly, since a design is made as such that the rotor 3 shaped in roundness rotates in the container chamber 2 of an outer casing 1 which is formed into a deformed circle. Moreover, the airtightness of airtight chambers 4 and 5 formed between the container chamber 2 and the rotor 3 is ensured, since close contact fittings 18a, 18b, 18c and 18d being in constant contact with the inner peripheral surface of the container chamber 2 of the outer casing 1 are provided on the outside of the rotor 3, and therefore suction, compression, explosion and exhaust are conducted smoothly and without fail. While the structure shown in the figure is so designed that suction, compression, explosion and exhaust are conducted twice during one rotation of the rotor, it may also be designed so that the suction, compression, explosion and exhaust are conducted once or three times or more during one rotation of the rotor.