Electric power is stored in a flywheel assembly, from a dc power buss, and supplied to the buss, through electronics associated with a motorgenerator, its rotor integral with a flywheel supported by magnetic bearings. The output restriction device includes an abnormality detection unit, and initial restriction unit, and a secondary restriction unit. The shaft is inserted in the shaft hole, and oriented so that a threaded hole in the center of the shaft can receive a threaded pin section of the locking pin.