The Red-billed Leiothrix Leiothrix lutea which was an introduced species from China had colonized in natural beech forests in Honshu and Kyushu, Japan since 1980’s. It was reported that some native bird species decreased where the Red-billed Leiothrix increased. On the other hand, Sika deer Cervus nippon have been rapidly increasing most area in Japan, which have occurred the change of bird communities through the alternation of natural vegetation. Thus, the author investigated the effect of the Red-billed Leiothrix on native bird community where the density of Sika deer had increased recently. Bird census had carried out by line-transect method in breeding seasons of 2003 – 2011 on Saorigahara, Kochi Prefecture. In the results, the Red-billed Leiothrix had decreased in 2011 after the increase from 2003 to 2007. On the other hand, the Bush Warbler Cettia diphone, which was a native species and had the nesting niche overlapped with the Red-billed Leiothrix, had decreased largely in 2009. The tree layer of the forest vegetation had not changed during the research period, though the coverage of the bush layer had largely decreased because most bamboo Sasamorpha purpurascens had died by the foraging pressure of Sika deer. These results suggested that both of the Red-billed Leiothrix and the Bush Warbler had decreased by the effect of the disappearance of the bush layer.