PURPOSE: To support casings elastically from the ground by providing the device with the plural number of rotors having a fly wheel which keeps the rotor rotating with an inertia force and an electric circuit which makes each rotor rotate in the opposite direction and increases or decreases the number of rotations of each rotor, keeping the number of rotations of each rotor equal.

CONSTITUTION: Casings 4, 5 are secured in a block 3. Since a rotor of each of the casings 4, 5 rotates in the opposite direction at the same rotational speed and the weight is the same, rotational torques generated by the rotors are offset. This means that no reaction force is generated in the block 3 due to the rotation of the rotors and therefore the swing of the block 3 due to the rototation of the rotors 9 is prevented. The block 3 is supported by a concrete bit with air springs and so the block 3 is kept from longitudinal and lateral vibrations due to an earthquake. Consequently, the swing of the entire electric power storage device can be prevented.