Sapium sebiferum, a bird-dispersed tree native to China, has become widely naturalized in Japan and is likely to pose a threat to indigenous ecosystems. To provide a scientific basis for the management of this species, we examined the process and limiting factor(s) of naturalization of this species at the Higashi-Hiroshima Campus of Hiroshima University, Hiroshima Prefecture. Numerous seedlings were observed in forest stands located about 50m from mature trees. During February-March, birds actively consumed the seeds. Seeds sown in an experimental field showed a high percentage of germination, but did not germinate until late May when leaf expansion of native deciduous trees had almost completed. In a field experiment in which potted seedlings were placed on forest floors under different light conditions, growth of the seedlings was suppressed severely under a closed canopy. These data suggest that the invasion of S. sebiferum to forests that are dominated by deciduous trees is restricted by the slow germination and high light requirements of the seedlings.