PURPOSE: To make rotating motion smooth and obtain high output by providing a combustion chamber on a rotor housing out of rotary driving range of a rotor and making a working direction of energy to rotating direction.

CONSTITUTION: An engine consists of a structure A made of a rotor housing having a combustion chamber and a rail, a rotor structure B configurating four cylinders which are compressors of combustion gas, pistons and combustion gas valves in cross shape and connecting respective units and a structure C covering the structure B. Energy generated in the combustion chamber is actuat ed to combustion gas valves and changed to rotating motion energy. The dis tance between combustion energy generating position and actuating direction and a rotary shaft is made larger, and combustion energy of four units is gener ated by one rotation in a small combustion chamber. Thus, rotating motion is interlocked smoothly and high output can be obtained.