coupled to the driven side are controlled so as to reciprocate between the pair of the outer magnets 1 coupled to the driving side as shown in fig5 a control mechanism 5 for the magnets 2 coupled to the driven side may comprise for example a spindle 51 to which the core 3 is coupled by the intermediary of an arm 52 so that the inner magnets 2 and the core 3 may reciprocate like a pendulum alternatively the control mechanism may comprise rail means 53 and the runner means 54 coupled to the core 3 movably engaged with said rail means 53 as schematically illustrated by fig6 in a sate that the pair of the outer magnets 1 face the pair of the inner magnets 2 in mutually parallel relationship respectively in the course of the rotation of the outer magnets 1 the outer magnets 1 cause the inner magnets 2 together with the core 3 to be attracted to the side in which the poles of the inner magnets 2 face the unlike poles of the outer magnets 1 and to be repulsed from the side in which the poles of the inner magnets 2 face the same poles of the outer magnets 1 see fig1 a c fig2 a c and fig3 a in a state that the pair of the outer magnets 1 face the pair of the inner magnets 2 in mutually orthogonal relationship the outer magnets 1 cause the inner magnets remain in equilibrium and consequentially the inner magnets 2 are located midway between the pair of the outer magnets 1 see fig1 b 2 b and 3 c in this manner this process is alternately repeated as illustrated by a through.