[Problem] To provide an antibacterial agent, by which the growth of Vibrio parahaemolyticus can be suppressed using a small amount of an active ingredient, and a method for suppressing a disease of a marine organism caused by infection with V. parahaemolyticus. [Solution] An antibacterial agent for suppressing the growth of V. parahaemolyticus, said antibacterial agent comprising at least propyl gallate. A method for suppressing a disease of a marine organism caused by infection with V. parahaemolyticus may comprise putting propyl gallate into rearing water.