Fujitsu is improving the usability of mobile phones based on human-centered design that involves the research, development, and evaluation processes, as well as the strengthening of human resources. Third-party research and creating Persona in the research process, user testing and heuristic evaluation in the evaluation process, and training sessions for strengthening of human resources are a part of this effort. In the development process, we applied our method previously devised for Web development called the Scenario-Based Walkthrough method, which allows those not specialized in usability to easily conduct evaluation at the design phase. We also prepared a usability dictionary that contains case studies of usability problems found in various evaluations. Through these approaches, we established a sustainable human-centered design process, which led to the improvement of product usability. This paper introduces an outline of each process and gives specific examples of improvement.