The key to success in business system development is to sufficiently elicit user requirements from the customers and to fully and correctly define the requirements analysis model that meets these requirements. Prototyping is recognized as an effective software development method that enables customers to confirm the validity of the requirements analysis model at an early stage of system development. However, the development process requires guaranteeing consistency between the system model and customer requirements that arise as a result of the confirmation. This paper proposes a method for the incremental validation of a Web application wherein a prototype system is automatically generated from a requirements analysis model based on UML (Unified Modeling Language). This model defines the interaction between a system and the user, in addition to defining the input/output data. Moreover, the automatic generation tool of the prototype system enables the developer to confirm the system image incrementally while developing the requirements analysis model in UML. We discuss the expressiveness of the generated prototype in comparison with the current group work support tool.