A web shopping mall is a powerful vehicle for attracting a lot of customers to the Internet. To analyze customer's purchasing pattern, a wide variety of web mining tools were used in literature. However, those tools suffer from the lack of considering cause-effect relationship between several factors consisting of the web shopping mall. In this sense, we propose using fuzzy cognitive map(FCM) to analyze customer's purchasing pattern. To add more rigorous analysis procedures, we introduce a concept of core FCM and unified FCM. To induce a meaningful FCM, we applied web log analysis, sorting out a set of useful causal relationships among the components of web shopping mall. Simulation results with unified FCM yielded that the proposed approach using FCM is robust and powerful for analysing customer's hidden purchasing pattern on a specific web shopping mall.