1. A sensor for automatic doors which comprises: means capable of taking an image of a predetermined area around a door; means for measuring a distance between the image pickup means and each object observed in the taken image of the predetermined area, the distance measurement means operating on receiving an output from the image pickup means; means for identifying the presence of an object which is neither a background object nor the door, the object identification means operating on receiving an output from the distance measurement means, wherein each object for which the distance from the image pickup means is measured is identified as a background object or an object other than the background object, based on the measured distance, and wherein the object other than the background object is identified as the door or an object other than the door; and means for outputting an object detection signal, the output means operating on receiving an output from the object identification means, wherein the object detection signal is produced, with a proviso that the object which is neither the background object nor the door is observed in the taken image of the predetermined area.