1. A data classification apparatus comprising: a separation surfaces set storage part configured to store information defining a plurality of separation surfaces which separate a feature space into at least one known class region respectively corresponding to at least one known class and an unknown class region, wherein each of the at least one known class region is separated from outside region by more than one of the plurality of separation surfaces which do not intersect to each other; and a classification part configured to determine a classification of a classification target data whose inner product in the feature space is calculable by calculating to which region of the at least one known class region and the unknown class region determined by the information stored in the separation surface set storage part the classification target data belongs.