1. A discriminator generation device comprising: an extraction unit which, using at least two pattern groups having patterns of different sizes for a detection object, extracts a feature quantity of the patterns configuring each pattern group; and a first generation unit which generates a first discriminator for detecting the detection object of a size corresponding to each pattern group in an image based on the feature quantity of the patterns configuring each pattern group; and a second generation unit which generates a second discriminator for a pattern group of a size different from the size corresponding to each pattern group based on the first discriminator generated by the first generation unit, wherein the second generation unit multiplies a determination threshold value of the feature quantity by a coefficient and corrects the determination threshold value of the feature quantity by the first discriminator generated by the first generation unit based on the size of the detection object.