Disclosed is a vehicle door driving apparatus provided with a driving mechanism and a rope member. The driving mechanism is fixed to an inner section of a vehicle door which opens and closes a door opening formed in a vehicle body, the driving mechanism including a motor and a drum which is rotationally driven by the motor. The rope member is wound around the drum and linked to the vehicle body, and rotation of the drum is transmitted to the vehicle body by way of the rope member which causes the vehicle door to open and close. The drum is disposed in the inner section of the vehicle door in such a way as to at least partially overlap in a dimensional space in the vehicle width direction defined by the external shape of the motor. The driving mechanism is provided with: a first small-diameter gear linked to the rotary axis of the motor in such a way that the first small-diameter gear rotates together with the rotary shaft of the motor; a transmission gear rotatable around the axis extending in the vehicle width direction; a sun gear rotatable around the axis extending in the vehicle width direction; a ring gear disposed on the same axis as the sun gear; and a carrier which has a planet gear that engages with the sun gear and the ring gear, and which is linked to the drum in such a way that the carrier rotates together with the drum.