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; wherein the detection candidate determination unit compares the parallax gradient of the pixel and a parallax gradient threshold corresponding to the acquired parallax of the pixel for each pixel in the parallax image, and determines whether the detection candidate is present from each of the pixels; wherein the parallax gradient calculation unit calculates and outputs the parallax gradient αi=ΔDisparity/ΔV, where V is an image ordinate; and wherein the parallax gradient threshold is determined based on a maximum gradient of the road surface.