A data accumulating section accumulates the data acquired from the monitored object. A reference space forming section forms a reference space based on the data accumulated in the data accumulating section. A data acquiring section acquires the data from the monitored object. A distance calculating section calculates a Mahalanobis' distance by inputting the data acquired by the data acquiring section into the reference space formed by the reference space forming section. An instruction accepting section accepts an instruction from a user, and decides procedures of operations of the reference space forming section, the data acquiring section, and the distance calculating section in response to the instruction.