1. An imaging apparatus which has two imaging units, comprising: a correlation value processor programmed to calculate a correlation value from two images captured by the two imaging units; a three-dimensional object determination processor programmed to determine an existence of a three-dimensional object from the two images; a region dividing processor programmed to divide an image region including the three-dimensional object into multiple regions; a relative speed processor programmed to calculate relative speed for every region of the multiple regions; a reliability calculation processor programmed to calculate a reliability of a determination of the existence of the three-dimensional object, based on the relative speed calculated for every region; a feature amount processor programmed to calculate a feature amount from an image including the three-dimensional object detected by the three-dimensional object determination processor; and a matching degree processor programmed to calculate a matching degree of the feature amount between a past feature amount which is stored and a current feature amount which is calculated, wherein the relative speed processor calculates the relative speed, based on the calculated matching degree.