A data classification apparatus for classifying plural input data into plural categories, in which the apparatus includes a prototype select unit for selecting the prototype of the category nearest to the input data that has been read, a prototype evaluation unit for evaluating whether the selected prototype is proper, a prototype addition unit for adding a prototype in the case where the selected prototype is not proper and an internal data correcting unit for correcting at least one of the prototype and an area determining parameter specifying the size of the category area for each category in the case where the selected prototype is proper. The size of the category area can be set for each category, and therefore, the data can be properly classified and the judgment accuracy is improved in an application to fault detection and fault diagnosis.