Leiothrix lutea, an alien bird species in Japan, is now dominant in the natural forests. The author first recorded it in 1989 at a mixed natural forest of Chichibu mountains, near the center of Honshu Island of Japan. We caught 605 birds in 37-time mist net surveys during 1989 - 95, and 608 birds in 19 surveys during 2006 - 09. All the surveys except once, were consisted by an evening and morning sessions with about ten 12m x 2.4m nets. Of those, L. lutea was three and 65 in the capture number in each term, and twice and 10 times in occurred frequency. While Cettia diphone, which may be a competitive species, was 224 and 110 birds, or 37 and 17 times in each term. L. lutea has apparently increased its number and become a dominant species in this forest undergrowth bird community, while C. diphone has decreased its density, though the latter is still one of the most dominant birds. Emberiza variabilis, a constant summer resident, was 45 and 22 birds or 22 and 10 times in frequency, and it had also decreased its density. It is not clear that L. lutea's has an impact on the two native birds, because there were other environmental changes such as the increases of temperature and Cervus nippon, and maybe the decrease of Sasamorpha borealis. The author thinks the ecosystem change during the last two decades is at least directional, and the alien bird plays a role.