A 12-years monitoring was conducted of a bamboo forest adjacent to a deciduous broad-leaved forest to clarify the bamboo stand dynamics during its expansion into an adjacent forest. The bamboo forest gradually expanded its area into the deciduous broad-leaved forest at an invasion rate of 7 m over 12 years. The density and aboveground biomass of bamboo living culms in the study plot increased. Conversely, the understory trees' density and species number decreased. The mean diameter at breast height (dbh) of living culms did not differ between the interior zone and the periphery zone of the bamboo stand through observation. Although increment number and aboveground biomass were constant in three innermost sub-plots, they were markedly increased in outer part, which increased understory trees' mortality.