PURPOSE: To raise the efficiency of energy conversion and to thereby lower the specific fuel consumption of an internal combustion engine, by disposing a cylinder annularly such that the axis of the cylinder is parallel to the turning shaft of a cylinder rotor, and shaping the cylinder rotor in a cylindrical form.

CONSTITUTION: A cylinder 1 is disposed in an annular form such that the axis of the cylinder 1 is parallel to the turning shaft of a cylinder rotor 2, and the cylinder rotor 2 is shaped in a cylindrical form. A piston arm 4 of each piston 3 is moved along a guide groove 6 to cause reciprocal movement of the piston 3 and to thereby repeat the cycle of suction stroke, compression stroke, expansion stroke and exhaust stroke. By designing the guide groove 6 appropriately, the stroke length and the stroke period of the four strokes can be selected in a desired manner. Thus, it is enabled to manufacture a highly efficient engine meeting the purpose of use of the engine.