into another force such as those indicated by the lines b 1 and b 2 the intersection is also varied thus a desired intersection can be produced it is naturally possible to produce a desired intersection by varying the line of magnetic force of the magnet the influence of the return spring of the piston on the pressing force of the piston working efficiency is represented in table 2 in this case the diameter of the piston is set at 50 mm and the stretching force of the return spring is set at 5 kg although the actual pressing force of the piston is the value calculated by subtracting 5 kg from the value shown in table 2 in consideration of a practical numerical value range for example if the supply pressure is 5 kg cm 2 the effective pressing force which is practically usable with respect to 100 kg which is the pressing force produced in other words the efficiency is 95 if a plurality of engine bodies are used by means of a crank mechanism since the backing motion is actuated by the advancing motion between each other the return spring may be dispensed with as desired fig3 shows another embodiment of the present invention a plurality of engine bodies 11 are provided in opposition to each other the magnet 7 is an integral disc shaped rare earth magnet and the magnet 8 is an integral annular rare earth magnet the piston 2 has a connecting rod 29 rotatably connected to the piston 2 by a pin 27 the connecting rod 29 is attached to an adapter plate 28 on which a blade or the like is mounted to the adapter plate 28 another connecting rod 30 is attached so as.