The present charging rate of a battery is accurately calculated. The calculation mode of the present charging rate is switched to a current integration mode during operation of a generator motor while the mode is switched to a voltage reference mode at a predetermined switchover timing. In the current integration mode, charge current of the battery is integrated and added to the initial charging rate and an integrated discharge current value is subtracted from the initial charging rate of the battery to calculate the present charging rate. In the voltage reference mode, the present charging rate is calculated based on a predetermined map and detected voltage of the battery. After calculation of the present charging rate according to the voltage reference mode, the calculation mode is restored from the voltage reference mode to the current integration mode and the suspension of operating the generator motor is cancelled.