Abstract: The present invention provides a structure of an engine which employs a cylinder block the cross-sectional configuration of which defines a predetermined curved surface and a piston having the same curved surface as that of the cylinder block, thereby enabling the arcwise reciprocating motion of the piston along the curved surfaces to be transmitted in the form of rotational motion for driving a shaft. Further, the engine is arranged as being a rotor composite engine in which the arcwise reciprocating motion obtained from the cylinder block and piston having such special configurations is transmitted to a connecting rod and further to a connecting rod body having a rotor journal and a rotor gear, thereby enabling the shaft driving force to be transmitted with high efficiency.