A planetary gear set may include a first rotation element fixedly connected to a first shaft, a second rotation element fixedly connected to a second shaft, and a third rotation element fixedly connected to a third shaft; a first motor which is mounted to supply power to the first shaft continuously; and a second motor which is mounted to supply power to the second shaft continuously, and the third shaft is connected to be selectively connectable to a transmission housing, and any two shafts of the first shaft, the second shaft, and the third shaft are configured to constrain rotations thereof to each other.