To perform cooling even during working, to prevent lowering of capacity of a damper, to perform cooling even during a stop, and to operate a brake.

A power from a power source 1 is fed to a motor 3 with a start/stop switch 2 closed. A flywheel 4 is rotated, the flywheel is contained in a case 5, the case 5 is coupled to a damper 7 through a shaft 6, and rocking is absorbed by the damper 7 and decreased. During CMG operation, a power is fed to a fan 10 from the power source 1, the increase of the temperature of the damper 7 is suppressed by the fan 10, and lowering of capacity of the damper 7 is prevented. During a stop of CMG, no power is fed to the motor 3 but the flywheel 4 continues rotation by inertia and is functioned as a generator. The fan 10 is driven by the power therefrom to cool the damper 7. Further, the fan 10 is worked as a resistance brake and the stop time of the CMG is quickened.