Provided are: an agent for preventing bacterial wilt disease, which agent can prevent the development of bacterial wilt disease caused by various different bacterial wilt disease strains, that is, the development of bacterial wilt disease in various different plant varieties; and a method for preventing bacterial wilt disease utilizing the agent for preventing bacterial wilt disease. The agent for preventing bacterial wilt disease comprises as an active ingredient a wilt bacterium (Ralstonia solanacearum) infected with the φRSM1-type filamentous phage and/or the φRSM3-type filamentous phage. The method for preventing bacterial wilt disease comprises the step of inoculating the agent for preventing bacterial wilt disease to a plant. The plant is preferably any one of tomato, potato, green pepper, eggplant, tobacco, capsicum, Japanese basil, Japanese radish, strawberry, banana, marguerite, chrysanthemum and sunflower.