1. A data classification method which classifies a plurality of data into a plurality of classification items based on a feature quantity included in the data, the method comprising: calculating, by a processor, an appearance probabilities in which training data including the feature quantity appears in the classification items in a distribution of the data; generating, by the processor, a rule having the feature quantity and a weighting of the feature quantity based on a plurality of the training data having the feature quantity based on the appearance probabilities; and classifying, by the processor, the plurality of data according to the rule.