PROBLEM TO BE SOLVED: To provide an antibacterial agent composition exhibiting excellent antibacterial action against an acetic-acid-resistant microorganism in an acidic composition, especially against an acetic-acid-resistant lactic-acid bacterium in an acidic composition, and to provide an antibacterial or storing method of the acidic composition.SOLUTION: The antibacterial agent composition for the acidic composition contains chitosan and thiamine dilaurylsulfate as active ingredients.