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 from the plurality of the captured images and acquire a provisional size of the target object included in the plurality of the captured images, wherein the second acquiring unit uses search windows having a plurality of search window sizes that are different from one another, scans each captured image, and acquires the provisional size of the target object included in a scanned captured image; a first setting unit configured to set at least one search candidate point in a vicinity of the provisional three-dimensional position in a real space; a second setting unit configured to set search windows used to search the target object, the search windows corresponding to each projection position at which the search candidate point is projected on each captured image, the search windows 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 evaluation values representing whether or not the target object is included inside the search windows; and a second calculating unit configured to calculate a three-dimensional position of the target object by using at least one search candidate point based on the evaluation values.