Provided is a novel lactic acid bacterium strain which is capable of suppressing production of volatile sulfur compounds by oral bacteria, has no cariogenicity and no causative role in infective endocarditis, and is safe in an oral cavity, and provided is an agent for preventing, improving and/or treating oral diseases and discomforts by use of the bacterial strain. An agent for preventing and/or treating an oral disease, containing, as an active ingredient, at least one lactic acid bacterium selected from Lactobacillus crispatus, Lactobacillus fermentum, Lactobacillus gasseri, and Streptococcus mitis and having all of the following properties (1) to (6): (1) having no ability to produce a volatile sulfur compound (VSC); (2) having no ability to produce water-insoluble glucan; (3) having adhesiveness to a tooth surface and/or an oral cell; (4) having a growth inhibitory effect on a bad breath-causing bacterium and/or a periodontal pathogen; (5) having no causative role in infective endocarditis; and (6) having no cariogenicity.