To take out and accumulate an inertia force in a piston reciprocating motion, as energy between permanent magnets or energy of springs, and to return the energy to a rotating force of an engine.

A crankshaft 9 within an engine body is extended, and a piston 5 and a cylinder 6 which interlock and synchronize with each other are provided. Permanent magnets 1 and 3 are attached on the piston 5, and permanent magnets 3 and 4 on the cylinder 6 for accumulating an inertia force in a piston reciprocating motion as a repulsive force or attraction force of permanent magnets. Alternatively, a spring 13 is arranged on the front side and rear side of the piston 5 (arrangement on either side is also possible), for accumulating as a compressive force or a tensile force of the spring, or a reduction gear is provided on the crankshaft 9, and the rotation is transmitted to the reduction gear and an inertia force of the piston 5 is accumulated, by a repulsive force or an attraction force between the permanent magnets provided there and the permanent magnets on the cylinder 6 side facing each other, which is thereby returned to a rotating force of an engine.