A normal revolution number calculation unit calculates a normal revolution number of a driving motor generator in every control period, based on a signal from a rotational position sensor. A moving average calculation unit calculates a moving average revolution number of the normal revolution number given in every control period. A predicted revolution number calculation unit determines whether or not the revolution number of the motor generator is in an increasing state, from the locus of the moving average revolution number. Determining that the revolution number of the motor generator is in the increasing state, the predicted revolution number calculation unit calculates a predicted revolution number based on respective moving average revolution numbers in the present and preceding control periods. The calculated predicted revolution number is set to be used as a control revolution number and output to a motor control unit and a torque command calculation unit.