An information acquisition unit acquires three-dimensional environmental information from a three-dimensional environmental information storage unit. A placement region detection unit uses the three-dimensional environmental information to detect a placement region where a target object is placed. A calculation unit calculates a size of a placement region in an image including the placement region. A viewpoint position determination unit determines, among a plurality of sensor positions where a sensor photographing the image related to image data can be placed, a sensor position where the placement region is larger than the placement region in the case of using image data of another sensor position, as an optimal viewpoint position. A sensor position control unit performs control so as to move the sensor to the optimal viewpoint position, and then performs control so as to start a search for the target object.