Abstract: The invention relates to a 4-phase rotary piston combustion engine. The embodiment of the engine consists of the engine body (1) where the main cylinder (2) is located; the piston (4) which revolves in the main cylinder and a cylindrical piston body is a fixed shaft (5). Thus the shaft functions as the driving shaft of the engine. The engine is operated by two valves (6, 9) which either open or close the space existing between the main cylinder surface and the piston body. The rotation of the piston is achieved when the gas mixture compressed into explosion volume is closed in between both the valves (6, 9) in such a way that also the piston stays between the valves. When the piston moves towards the valves in front (9) it pushes the combustion gas from the front side of the piston to its back side through a groove (13) located in the surface of the main cylinder. When the combustion gas has moved to the back side of the piston, the explosion takes place and the valve in front of the piston opens.