The present invention inhibits biofilm formation by Streptococcus mutans and treats tooth decay by using a vegetable lactic acid bacterium belonging to the genus Enterococcus, said vegetable lactic acid bacterium being isolated from rice hull or hulled rice. To treat tooth decay, the aforesaid vegetable lactic acid bacterium is contacted with S. mutans in the subject's oral cavity. Thus rice hull can be more easily and effectively utilized.
