Fluctuations in rotating speed or in torque at the time of switching between series drive and parallel drive operations are prevented. When switching from series drive mode to parallel drive mode, the rotating speed Nb of a motor B is detected and a rotating speed command is transmitted to match the rotating speed Na of a motor A with the rotating speed Nb of the motor B. When the actual rotating speed Na of the motor A has become equal to the rotating speed Nb of the motor B, a torque command which causes torque Teg of an engine (1) to become zero is transmitted and, then, a command for engaging a lock-up clutch is transmitted. When switching from the parallel drive mode to the series drive mode, the engine (1) torque is gradually reduced and the motor B is controlled to output a torque corresponding to the amount of reduction in the engine (1) torque. The rotating speed Na of the motor A is matched with the rotating speed Nb of the motor B and the lock-up clutch is disengaged when the engine (1) torque has become zero.