PURPOSE: To make it possible to store energy and liberate it outside when required, by having kinetic energy absorbed into a torsion spring or tensile/compression spring made of a quasi-elastic metallic material.

CONSTITUTION: When a coil 2 is energized, friction plates 3 and 4, which form a clutch together with the coil 2, are pressed to make contact with each other, whereby the rotation of an input shaft 1 is transmitted through the friction plates 3 and 4 and gears 5, 19 and 15 to twist a torsion spring 14 made of a quasi-elastic metallic material so as to store energy. By de-energizing the coil 2 and energizing a coil 7, the torsion spring 14 can be held in the twisted state. By de-energizing the coil 7 and energizing a coil 12; friction plates 10 and 11 are pressed to make contact with each other, and the torque of the torsion spring is returned through the gears 17 and 9 and friction plates 10 and 11 to the input shaft 1.