1. A motion vector detection device comprising: hardware, including a processor; an edge extraction unit implemented at least by the hardware and that extracts edge points from each of a current frame and a previous frame, the previous frame being a frame preceding the current frame; a feature point extraction unit implemented at least by the hardware and that extracts, from each of the previous frame and the current frame, a feature point which is an edge point whose positional relationship with another at least one edge point within a frame is uniquely determined within the frame, among a plurality of edge points extracted by the edge extraction unit; a feature point pair extraction unit implemented at least by the hardware and that extracts, as a feature point pair, a pair of feature points, the positional relationship of which is common, among pairs of a feature point of the previous frame and a feature point of the current frame; a motion vector calculation unit implemented at least by the hardware and that calculates, as a motion vector, a difference between coordinate values of the feature point pair extracted by the feature point pair extraction unit, wherein the feature point extraction unit extracts the feature point using a hash value of a distance between edge points as the positional relationship, including a hash table for each frame, the hash table having registered therein a group of a hash value, an appearance frequency, and a final appearance coordinate value, without a list of appearance coordinate values, of an edge point having the hash value, wherein the edge extraction unit calculates the hash value for each edge point extracted from the previous frame and the current frame, updates the appearance frequency corresponding to the hash value in the hash table, and stores the appearance coordinate of the edge point to the final appearance coordinate value corresponding to the hash value, the feature point extraction unit extracts, as a feature point, an edge point paired with a hash value corresponding to an appearance frequency of 1 in each of the hash table of the current frame and the hash table of the previous frame, and the feature point pair extraction unit extracts, from the previous frame and the current frame, a feature point pair that satisfies a condition that all absolute values of differences between pixel values of each component of the feature points pairs are equal to or smaller than a threshold, in addition to a condition of having the same positional relationship with another edge point.