PURPOSE: To prevent a mechanical loss due to eccentricity by engaging a right circular rotor chamber with a rotor provided with combustion chambers deeper in a rotational direction on a periphery face fixed on an concentric output shaft for providing each hole for suction, ignition and exhaustion in the rotor chamber.

CONSTITUTION: When a rotor 3 is rotated clockwise, combustion chamber 5W8 pass a fuel supply hole 9, an ignition hole 10 and an exhaustion hole 11 one by one. When the combustion chambers 5W8 come close to the supply hole 9, a piston 14 is displaced by pressed to the side of the rotor chamber 2 and an injected fuel-air mixture is compressed gradually to fill the combustion chambers 5W8. When the rotor 3 is further rotatd and the piston 14 is returned to the original place by a spring, the rotor chamber 2 side of the supply hole 9 becomes a negative pressure, and then the fuel-air mixture is sucked and injected into the ignition hole 10 from a pipe 24. Then, an ignition plug 23 sparks to fire the fuel-air mixture inside the ignition hole 10, and the fire runs from shallow place to deeper place in a high speed inside the combustion chambers 5W8 to rotate the output shaft 4 clockwise. In this way, the rotational power of the rotor can be transmitted to the output shaft without generating any mechanical loss.