PURPOSE: To optimize the rotational frequency of a turbine in a motor mode for preventing electric power from useless consumption by judging a change in the rotational frequency of the turbine while supercharging air pressure has the same pressure value for a predetermined time and setting detected supercharging air pressure to the upper limit value of supercharging pressure leak while the rotational frequency of the turbine is rising.

CONSTITUTION: A dynamo-electric machinery 7 is mounted on a rotary shaft 6c of a turbocharger 6. DC electric power from a battery 5a is converted to be supplied to the dynamo-electric machine 7 when the torque of an engine 1 is increased to drive a compressor 6a as a motor mode. On the other hand in a generator mode, electric power generated by the dynamo-electric machine 7 is charged to a battery 5a through an electric power converter 7a. Then, a boost pressure sensor 1d and turbine rotational frequency sensor 6d are provided to detect a change in the turbine rotational frequency when boost pressure shows the same pressure value for a predetermined time and set the detected boost pressure as the upper limit value of supercharging pressure leak while the turbine rotational frequency is rising, and when the upper limit value falls below a regular lower limit value, an alarm is generated.