A patio door operating unit comprises a housing which mounts on the floor beside the location that the door occupies when the sliding door is in its open position. Within the housing in an endless belt which engages the door adjacent its bottom and is trained for movement along a path at the bottom of the door. This belt is driven by an electric motor. There are limit switches to stop the electric motor at the open and closed positions of the door. The belt is tightened by a solenoid connected to the motor circuit so that when the electric power fails the belt is loose and the door can be moved by hand.