To make the high rotation of the flywheel easy, more specifically, to reduce rotational resistance, to improve the dissipation of internal heat, and also to attain the high performance of a generator motor for giving and taking the rotational energy of the flywheel.

In a closed space composed of a casing 12 and a mount 1, a hydrogen gas 24 is enclosed by different pressure from atmospheric pressure. The pressure is maintained in a predetermined safety range in a form that spontaneous emission is carried out from a hydrogen storage alloy 25 prepared in the casing 12. Also, other than the flywheel, in the closed space, an inverter, a dual chopper, a field control FET, and a controller for controlling these are incorporated.