1. An object detection device comprising: an outline image extraction unit extracting an outline image from an input image; a feature vector calculation unit calculating a feature vector from the outline image by using a histogram of oriented gradients (HOG) representing a frequency distribution of gradient vectors with respect to pixels of the outline image, and pixel coordinate information varying according to a spatial distribution of the gradient vectors; and an object judgment unit judging a target object corresponding to the feature vector with reference to pre-learned data.