1. A data classifying apparatus comprising: a class constructing section that classifies model data groups each comprised of a plurality of items of model data according to predetermined item, and thereby classifies the model data groups into classes each having a smaller distribution than a category that an application requires; a representative value calculating section that calculates a representative value of each class using data of respective one of the model data groups that is classified into the class; a class determining section that compares input data input from an outside with the representative value, and that determines a class with the representative value having a highest degree of similarity with the input data; and a category determining section that obtains the category to which the input data belongs using the determined class.