1. An object detection apparatus comprising: a memory storing instructions; and at least one processor configured to process the instructions to: indicate a part area in a plurality of images including an object to be detected, the part area being an area including a target part, the target part being one of parts forming the object to be detected; generate an appearance probability distribution and an absence probability distribution of the part area based on an appearance frequency of the part area associated with positions of the part area in the plurality of images; and determine an area including the object in an input image with reference to the appearance probability distribution and the absence probability distribution of the part area.