An application of a method for prioritizing requirements to an actual project is reported. The project where one of the authors participated as a project member developed in-house software development support tools based on a software product line. In the development of a software product line, a project needs to evolve core assets in accordance with changes to the environment, the market, and technology. The concerns of stakeholders may also change the process of evolving core assets over the years, and even if stakeholders change, the concept of the target product line should be maintained. In order to effectively evolve core assets, it is important for the project to prepare and utilize a standardized method for prioritizing requirements. In this paper, we analyzed the evolution of core assets in relation to an actual project. Tacit knowledge for prioritizing requirements was extracted. Such knowledge was made explicit and defined to develop a method for prioritizing requirements. The method consists of the rules and processes for applying the rules. We also defined a meta-model for prioritizing requirements and incorporated the concept of the improvement of rules into the meta-model. According to the evaluation of the method, the following issues were clarified: (a) different stakeholders smoothly and efficiently reached agreement using the method, and (b) the method is effective for reducing lead time and costs for defining requirements.