When a high-speed train enters a tunnel equipped with branches, micro-pressure waves emit from the portals of the branches as well as from those of the main tunnel. They can cause environmental problems related with high-speed railways, especially in urban areas. In this study, we investigate a countermeasure against the micro-pressure wave from the portals of the branches. It is shown that a branch divided in two parts by a partition inside near the portal with the end of one part closed and that of the other open, can be effective to reduce the magnitude of the micro-pressure wave.