PURPOSE: To prevent the distortion of a rotor caused by a thermal imbalance by not tripping a driving machine but making the machine to continuously rotate a rotor until the cooling liquid in the rotor is vaporized when a superconducting generator is tripped due to the occurrence of an accident.

CONSTITUTION: At the time of tripping a superconducting generator 11 due to the occurrence of an accident, a generator breaker 13 and excitation breaker 18 are opened and the supply of a cooling liquid to a superconducting field winding 2 is stopped. Then a driving machine 12 is not tripped but operated at a high speed to rotate a rotor 1 until the cooling liquid in the rotor is completely vaporized so as to prevent the occurrence of an unbalanced state of the winding 2 cooling liquid in the rotor 1 by utilizing the centrifugal force of the rotor 1. Therefore, the occurrence of distortion of the rotor 1 due to a thermal imbalance can be prevented.