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, and classifying that the classification target does not pertain to a specific category in case that the addition value is smaller than a threshold, wherein calculation of the addition value is carried out by adding function values having positive values, then adding function values having negative values, and the classification target is classified as not pertaining to the specific category, without adding the remaining function values, in case that the addition value has become smaller than the threshold.