In a system for automatically opening or closing a sliding door or the like for a vehicle, an undesired clamping of an object between the door and the vehicle body, resulting in increased tension force in the wire moving the door, is sent early and relieved quickly. The wire (4) is solidly secured to the sliding door and wound onto a drum (10) to open or close the door by a motor driving the drum, and two tensioners (11a and 11b) are provided respectively on opposite sides of the drum and between the drum and door. Each sensor has a movable part (19) positioned in accordance with the tension force in the wire passing through the tensioner, sensor plates (20) are provided on the movable parts of the tensioners, and magnetic sensors (21) are provided to detect the moving speeds of the plates. When the moving speeds of the plates are larger than a predetermined value, preset in advance, in cases other than when the door is at a fully closed or fully opened condition, a controller concludes that some object has been clamped between the door and the vehicle body and immediately relieves such clamping.