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.