1. A training apparatus comprising: a training sample storage unit that stores therein a plurality of training samples that are classified into C (C≧3) categories, wherein categories of the plurality of categories are different from each other; a selecting unit that performs, N (N≧2) times, a selection process of selecting a plurality of groups each including one or more training samples from the training sample storage unit, the one or more training samples included in each of the plurality of groups being selected at random; and a training unit that trains, each time the selection process is performed, a classifier Fi(x) for classifying the plurality of groups selected in the selection process, where 1≦i≦N, and generates an evaluation metric, wherein the evaluation metric is an array of classifiers F1(x), F2(x), through FN(x), the evaluation metric outputs a feature value v(X) for an input data, the feature value v(X) is an array of elements of an evaluation si, the evaluation value si is an output of the classifier F i (x) for the input data, the evaluation si is a vector, and the feature value v(X) indicates to which category the input data belongs among the C categories and one or more non-C categories.