This chapter describes the framework of knowledge discovery process in sales data and how the data mining system is applied to the data in the real business world by using the domain knowledge. First the framework of the knowledge discovery process in databases is reviewed. It is not clear how users construct the actual data mining process and use the domain knowledge in the existing models. We propose a two-dimensional matrix of knowledge for analysis of sales data to understand the knowledge discovery process from purchase history. Then we distinguish the data mining process from the creation of business action. We point out that efficient knowledge discovery can be achieved by intensively introducing domain knowledge of experts to the creation of business action.