Disclosed is an antibacterial agent having ameliorating and prophylactic effect on periodontal diseases and dental caries, which is characterized by the inhibitory effect of a sugar alcohol on proliferation of periodontal disease-causing bacteria and dental caries-causing bacteria. Specifically disclosed is an antibacterial agent for inhibiting proliferation of periodontal disease-causing bacteria, which contains a sugar alcohol, in particular lactitol or maltitol, as an active ingredient.