In recent years, it has become increasingly important to maintain the architecture in product line development (PLD), mainly due to rapid changes in market requirements and the technical environment. In PLD, architecture maintenance is a more complicated and difficult process than that in the case of conventional software development because of the scalability problems, i.e. it is necessary to manage variabilities considering not only the implemented architecture that constrains a concrete software implementation but also the reference architecture that constrains the implemented architecture. Although architecture refactoring is one of the important techniques used for maintaining architecture, existing architecture refactoring techniques mainly focus on the implemented architecture and are not sufficient for PLD. In this paper, we propose an architecture refactoring method that considers both the implemented and the reference architecture, in which we utilize the portfolio of problem factor to examine the strategy of architecture maintenance. The method is examined in terms of an actual project involving an embedded consumer product, and the result is compared with the results obtained in that project.