To provide a practical engine having a smaller size, a lighter weight, high output and low fuel consumption by achieving perfect circular rotation while eliminating the reciprocating action of a main shaft and a piston.

A fixed cylinder chamber 5 is internally shaped into a circular tube and arranged so that a core piston 15 is perfectly circularly rotated in its space. Three heads 16 of the core piston are each arranged on the outer periphery at 120° from the center of the core piston 15. Thus, a regular triangle having 60° formed by the three heads 16 of the core piston can keep a gravity center in such a manner that vectors as three centrifugal forces from the center of the triangle as the center of the piston are well balanced, and so rotating output is obtained which is stable in a range from low speed rotation to high speed rotation and less in energy loss.