Recently the needs for keeping existing customers is increasing in the field of marketing. So, the customers needs to be classified by groups and the differentiated responses to the specified customer groups are demanded. In this paper, we implemented a system that classifies the customer groups using the neural network, and classified the purchasing patterns among customer groups. Empirically examining the association rules between two groups, we could find out that similar rules exist between them. So, it is important that customers should be classified into the excellent customer group and the general group for the decision making of marketing. This paper shows that the efficiency of the differentiated marketing can be maximized by raising the correctness of the expectation in the classification of customer groups.