1. A category classification method, comprising: calculating function values corresponding to a relationship between a classification target and support vectors that contribute to a classification boundary, calculating an addition value in which the function value for each support vector has been added, wherein calculation of the addition value is carried out by adding function values having positive values, then adding function values having negative values until the addition value becomes smaller than a threshold, or no more function values remain, and the classification target is classified as not pertaining to the specific category in case that the addition value has become smaller than the threshold.