To bridge a wide gap between the end users and the requirements engineers, we propose a business flow diagram for acquiring users' requirements of the object oriented software development in the business application domain. Each field of this diagram shows either a role or a responsibility of a particular person or an organization. This paper proposes a development method that the engineers acquire the requirements by using our diagrams. We have implemented a supporting tool based on this study for collaborating the requirements engineers with their users. At first, the end users of an information system to be developed draw diagrams representing the flows of information and physical objects in their work from their own points of view. Sometimes the engineers write them with the users. If all users submit their diagrams, then our tool collects them and constructs a total diagram. The requirements engineers analyze the total diagram for improving the business flow. After the engineers complete this diagram, our tool can automatically transform it into an initial version of the class diagram. We show the effectiveness of our approach with some experiments. Comparing the related works, we discuss some issues of the practical aspects of this proposal.