To protect an inverter circuit or an electricity-storing means if an electric motor becomes uncontrollable and operates as a generator while it is rotating at high speed.

When an assisting motor 40 becomes uncontrollable while a driving shaft 22 is rotating at high speed, system main relays 94a, 94b are turned off to break a battery 94. Electric power regenerated by the assisting motor 40 via a three-phase full-wave rectification circuit consisting of the diodes D11 to D16 of a second drive circuit 92 is consumed by letting a clutch motor 30 operate as an electric motor by controlling the 'on-off' of the transistors Tr1 to Tr6 of a first drive circuit 91. As a result, the overcharging of the battery 94 by the electric power regenerated by the assisting motor 40 can be prevented. Also, the damage to the capacitor 93 becomes preventable by keeping the counter electromotive voltage of the assisting motor 40 less than the withstanding voltage of a capacitor 93.