In the user interface design of electric appliances, we propose a method to measure the consistency of the designer's mental model and the user's one, and that evaluate the validity of the user interface based on the results. The mental models are image formed in our head that affects human perception and decision making. As these are built from our experience and learning, individual results may vary. Thus, a gap in the mental model between designer and user is one of the factors in usability problems. In the method of the paper, the mental model is quantified by comparing dendrograms that classified product functions. The designer’s dendrogram is generated automatically from the design information by the clustering algorithm, and the user’s one is generated from the card sorting test. The product functions with a large gap in the mental model are identified based on the quantified results, and the designer can review the validity of the user interface. We applied the method to the user interface design of a digital camera and the results of experiments suggested the properness and effectiveness of the method.