Generally, end-users do so various kinds of operations on Web application systems that the developers often can not expect the effect on the system. However, it is very important that validity of user input is checked beforehand to avoid failures of service caused by unexpected user operation on Web application systems. We have proposed a method of model-driven requirements analysis using Unified Modeling Language (UML). The main feature of our method is to automatically generate a Web user interface (UI) prototype from UML requirements analysis model (RA model) so that we can confirm validity of input/output data for each page and page transition on the system by directly operating the prototype. The weakness of current our method is that there is a possibility that customers and developers miss such troublesome unexpected user operations because confirmation is only based on viewing of them. This paper proposes the improved method from the viewpoint of validity and usability so as to avoid failures of service which is caused by unexpected user operation on Web application systems.