To provide a customer data analyzing method by a database of purchase data, etc., in order to efficiently performing a marketing approach because maintaining significant customers whose purchase amount is large and increasing customer purchasing a lot are important for a dealer.

This method constructs a purchase history database of the customer and evaluates the contribution of the customer by including not only the item of a purchased amount from it but also items such as the number of yearly average purchasing times, the number of continuously purchasing years, an average purchase discount rate, the number of purchasing sales areas and the number of average days to the next purchase.