Usability is a crucial characteristic for Web applications. A user often re-inputs values because of mistakes or changes of mind during operation. Therefore, an Input Save Function (ISF) is useful to temporarily save a user's inputs. Whether an ISF can be implemented properly into an application depends on the results of analysis and design phases where CRUD (create, read, update, delete) timing of inputs, screen transitions, security and so on are determined. However, a method to aid in the design and analysis of ISFs is not presently available. In this paper, we propose a method to evaluate Web application usability based on the number of input items and input value lengths which are affected strongly by the ISF used. To achieve this purpose, methods to express screen transitions and user operation scenarios in UML are also proposed. The effectiveness of our method is demonstrated by using an actual e-commerce website.