1. An object detecting device, comprising: at least one wireless tag reader for detecting 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 data relating to the object ID information detected by the wireless tag reader, human position data measured by the human position sensor, and image data picked up by the image-pickup device, while associating each piece of the data with obtained time thereof; and an object detection means for detecting ID information and a position of the object based on the data relating to the object ID information, the human position data, and the image data, stored in the storing means, wherein when the object detection means determines that data relating to first object ID information and data relating to second object ID information corresponding to first time and second time on which the human position data is obtained respectively, among the data relating to object ID information 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 and second image data corresponding to the first time and the second time respectively, and in the calculation, the object detection means calculates a difference for an area determined based on the human position data from the first time to the second time, 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.