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 generation unit which generates a 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 discriminator for a pattern group of a size different from the size corresponding to each pattern group based on the discriminator generated by the generation unit.