1. An image processing apparatus comprising: a holding unit configured to hold a positional relationship of a plurality of image capturing units; an acquisition unit configured to detect objects from respective images captured by the plurality of image capturing units and acquire positions of the objects on the captured images and geometric attributes of the objects; a selection unit configured to select a combination of objects to be associated, based on the positional relationship and the positions of the objects on the respective captured images; a determination unit configured to determine a consistency of the selected combination based on a degree of similarity between the geometric attributes of the respective objects belonging to the combination; an associating unit configured to associate the objects in the combination based on the consistency of the combination; and an estimation unit configured to estimate a three-dimensional position of the objects based on the positional relationship and the positions of the detected objects.