1. An object detecting device, comprising: at least one wireless tag reader for detecting object ID data relating to object ID information of an object to which a wireless tag is given in a space where the object is searched; at least one human position sensor for measuring a position of a human handling the object in the space; at least one image-pickup device for picking up an image of a part or a whole of the space; a storing means for storing the object ID data, human position data measured by the human position sensor, and image data picked up by the image-pickup device, while associating a time with the object ID data, the human position data and the image data, wherein the time indicates when the object ID data, the human position data and the image data were obtained; and an object detection means for detecting object ID information and a position of the object based on the object ID data, the human position data, and the image data, stored in the storing means, wherein when the object detection means determines that first object ID data corresponding to a first time that first human position data is obtained and relating to first object ID information and second object ID data corresponding to a second time that second human position data is obtained and relating to second object ID information, among a set of object ID data in which a human position measured by the human position sensor is included in a detection range of the wireless tag reader, are different from each other, the object detection means calculates a difference between first image data corresponding to the first time and second image data corresponding to the second time, and in the calculation, the object detection means calculates a difference between the first image data and the second image data for an area determined based on the first human position data and the second human position data, and then, detects a position of the difference area in image data or a position obtained by converting the position to a position in the space where the object is searched, as a position of the object.