1. An object detection apparatus comprising: at least one processor and memory coupled to each other and cooperating to act as: an image obtaining unit configured to sequentially obtain a plurality of frame images; a detection unit configured to calculate a detection likelihood from an image pyramid generated by enlarging or reducing each of the plurality of frame images, and to detect a target object from each of the frame images on the basis of the detection likelihood; a tracking unit configured to calculate a tracking likelihood of the target object from each of the plurality of frame images, and to track the target object over the plurality of frame images on the basis of the tracking likelihood, and, a tracking likelihood conversion unit configured to convert the tracking likelihood of the target object so as to be subject to a likelihood calculation processing in a detection processing, on the basis of position information and size information on the target object, wherein the detection unit detects the target object from the frame images obtained by the image obtaining unit on the basis of the converted tracking likelihood of the target object, and the detection likelihood of the target object that is calculated by the detection unit from the frame images.