1. An object detection apparatus comprising: a detector that detects a candidate object from an omnidirectional image, the candidate object being a candidate of a detection target; a detection region former that forms a candidate detection region surrounding the candidate object, the detection region having a predetermined shape that has a height and a width with respect to predetermined axes; a storage that stores information of a standard detection region having a height and a width of the detection target with respect to the predetermined axes; a transformer that rotates the standard detection region according to axes defined by a positional relationship between a center position of the omnidirectional image and the position of the candidate object, and generates a projected standard detection region by projecting edges of the rotated standard detection region on the predetermined axes; and a determiner that determines whether or not the candidate object is the detection target based on a difference between a size of the candidate object detection region and a size of the projected standard detection region.