1. An object detection apparatus that detects information regarding an object comprising a plurality of parts articulated by joints, from image data that images the object, the object detection apparatus comprising: a part candidate extraction section that extracts position candidates of adjacent two of the parts from the image data, using constraint information of a reference model of the object; a joint angle extraction section that extracts an angle of a joint that articulates the two parts, from the extracted position candidates, and outputs positions and directions of part axes of the two parts, the part axes forming the extracted angle of the joint; a joint position extraction section that extracts a position of the joint of the object, from the positions and directions of the part axes acquired from the joint angle extraction section; a reference ratio joint position acquisition section that acquires a position of one of joints of the reference model, the one corresponding to the joint that articulates the two parts; and a part length ratio estimation section that detects, as the information regarding the object, a ratio of: a length of a given part of the object; to a length of a part of the reference model corresponding to the given part, on a basis of the position of the joint of the object and the position of the joint of the reference model, wherein the part length ratio estimation section corrects the constraint information on a basis of information regarding the estimated ratio.