A control device controls opening/closing of a sliding door of a vehicle by a motor in cooperation with a drive device that opens and closes the sliding door, and applies proportional plus integral plus differential control to motor speed based on a driving pattern of the sliding door. When a difference between a door speed and a motor speed exceeds a threshold, the control device judges that the vehicle is parked in a tilted state, and adjusts a sum of a proportional element, an integral element, and a differential element of a proportional plus integral plus differential control to a vehicle-tilted time element.