1. An image processing apparatus comprising: an object defining unit configured to define specific information specifying a specific object that can occlude a target object to be observed, and to define a three-dimensional model of the target object; an image obtaining unit configured to obtain a captured image of the target object captured by an image pickup apparatus; an occluding region detector configured to detect an occluding region including the specific object from the obtained captured image by specifying the specific object using the defined specific information; an occluding region extending unit configured to extend the occluding region outward by one or more pixels so as to form an extended occluding region; an image feature detector configured to, after the occluding region extending unit has extended the occluding region, detect image edge features of the target object from a region of the captured image other than the extended occluding region, such that the image edge features of the target object detected by the image feature detector exclude image edge features of the specific object; and a calculating unit configured to calculate a position and orientation of the target object in the captured image on the basis of correspondence between the image edge features detected by the image feature detector and features of the three-dimensional model of the target object.