1. An object detection device comprising: a parallax calculation unit that generates a parallax image based on a plurality of images; a parallax gradient calculation unit that, for each pixel in the parallax image, calculates a parallax gradient for each pixel based on the parallax of a predetermined pixel and a pixel that is adjacent to the predetermined pixel; a detection candidate determination unit that determines whether a detection candidate is present based on the parallax gradient of each pixel; a voting unit that carries out a histogram vote using each of the pixels that are determined to be detection candidates and generates a histogram frequency for each of the pixels when the detection candidate is determined to be present; and an object detection unit that detects presence of an object and a position of the object based on the histogram frequency.