A capacitance sensor with a directionality characteristic in the direction of its detection surface is incorporated in a detector for detecting an object that may be feared to or about to be caught by a closing door such as a slide door of a vehicle. The capacitance sensor has a main body enclosing detection electrodes, an insulating material placed in between and a shield electrode serving to provide the directionality. A water-repellant finish is provided at least on a portion of the outer surface of the main body including the detection surface for preventing water drops from remaining or becoming larger. The detection surface may be unevenly shaped with protrusions for making it difficult for water drops to remain attached.