PURPOSE: To reduce the energy loss of three motors when the motors are rotated at low speeds by connecting the motors to each other through a differential gear and fetching one of the rotating shafts of the motor as an output.

CONSTITUTION: The output shafts 4, 5, and 6' of three motors 1, 2, and 7 are connected to each other through a differential gear 3 so that the difference between the rotating speeds of the motors 1 and 2 can become the rotating speed of the motor 7. Whether the motors 1, 2, and 7 are driven as motors or regenerative generators are respectively decided by means of driving regenerative circuits SW1, SW2, and SW3. When, for example, the motors 1 and 2 are respectively used as a driving motor and generative generator and the output shaft 6 of the motor 7 is used as a power output shaft, the reduction of the rotating speeds of the motors 1 and 2 is not required and the occurrence of energy loss associated with a low rotating speed can be avoided even when the rotating speed of the output shaft is low, because the difference between the rotating speeds of the motors 1 and 2 becomes the rotating speed of the motor 7.