1. An object detection apparatus that detects a target object from image data, the apparatus comprising: an image obtaining unit adapted to obtain image data by capturing an image in a predetermined direction from a predetermined viewpoint; a feature extraction unit adapted to extract a feature amount for each of partial regions that constitute the image data obtained by the image obtaining unit; a storage unit adapted to store, as a background feature amount for each of the partial regions, a feature amount extracted in advance by the feature extraction unit from background image data that has been obtained by the image obtaining unit and does not include the target object; a first object region determination unit adapted to determine, for each of the partial regions, whether or not the partial region is an object region including the target object by comparing a current feature amount extracted from the partial region by the feature extraction unit with the background feature amount; a parameter deciding unit adapted to decide a parameter for each of the partial regions based on a plurality of results of determination by the first object region determination unit; and a second object region determination unit adapted to determine, for each of the partial regions, whether or not the partial region is the object region based on the parameter decided by the parameter deciding unit and the results of determination by the first object region determination unit.