A medical or dental material can be prepared by using, as an active ingredient, at least one of a protamine, and derivatives thereof and hydrolysates thereof, or a complex of at least one of a protamine, and derivatives thereof and hydrolysates thereof with an anionic high molecular weight compound. As a result, an antibacterial agent against periodontal disease-causing bacteria can be provided, which is highly safe without concern of a side effect. A medical or dental material can be also provided, which exhibits antimicrobial activity on periodontal disease-causing bacteria and additionally has insolubility in water and good formability.