1. An image processing device comprising: a first acquiring unit configured to acquire a plurality of captured images by imaging a target object from a plurality of positions; a second acquiring unit configured to acquire a provisional three-dimensional position of the target object and a provisional size of the target object; a first setting unit configured to set at least one search candidate point in a vicinity of the provisional three-dimensional position; a second setting unit configured to set a search window used to search the target object for each projection position at which the search candidate point is projected on each captured image, the search window having a size that is based on the position of the search candidate point and the provisional size; a first calculating unit configured to calculate an evaluation value that represents whether or not the target object is included inside the search window; and a second calculating unit configured to calculate a three-dimensional position of the target object based on the evaluation value.