PURPOSE: To double output and to reduce fuel by half by reciprocating annular working elements by the reciprocating motion of a piston, and forcing rotors to rotate on their axes and revolve, thereby rotating an output shaft through a crank-like member.

CONSTITUTION: Cylinder blocks 3, 4 are fixed to both sides of an enclosure 2 in an internal combustion engine 1, and plural signals 8, 9 and 10, 11 are disposed in the respective cylinder blocks 3, 4 in such a manner as to freely slide. Outer peripheral flange portions 14a, 19a of annular working elements 14, 19 mounted on the outer periphery of disc portions 14b, 17b of rotors 12, 17 respectively having rolling gear portions 12a, 17a through bearings 13, 18 are respectively inserted in rod portions 8aW11a of pistons 8W11. Further output shafts 28, 29 rotatably supported through bearings 26, 27 are provided on both ends of the enclosure 2, and fitting shafts 16, 21 are mounted on the inner ends of the respective output shafts 28, 29 at eccentric positions of discs 30, 31.