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 plurality of data, the method comprising: calculating, by a processor, for each feature quantity, an appearance probabilities in which first training data including the feature quantity appears in the plurality of classification items based on a distribution of the feature quantity included in the plurality of data with respect to the plurality of classification items, the first training data being included in a first training data group and being classified into the plurality of classification items; generating, by the processor, a rule having the feature quantity and a weighting of the feature quantity, based on a second training data group including a plurality of second training data of which the feature quantity is selected among the training data of each first training data included in the first training data group based on the appearance probabilities; and classifying, by the processor, the plurality of data according to the rule.