A two-way clutch is provided between an engine and a generator motor. Further, a battery and a cargo handling pump are provided. The generator motor is set to either a generator mode or an electric motor mode. When a cargo handling load (detected by a sensor) is small, the two-way clutch is set to a first state in which transmission of driving force from the engine to the generator motor is allowed and in which transmission of driving force in reverse is prevented. Further, when the engine is stopped or idled, the cargo handling pump is driven by the generator motor in the electric motor mode. When, during the above operation, the load increases to a level above a predetermined value, an increase in output of the generator motor and an increase in engine RPM are started, and when the engine RPM becomes equal to the RPM of the generator motor, which previously increased, the two-way clutch in the first state is connected, and the cargo handling pump is driven by the engine.