Provided is a novel periodontal disease-preventing agent having the advantage of having low risk of the emergence of resistant bacteria, in comparison to existing antibacterial agents. An oral cavity composition containing a cold water-extract of Brassica rapa var. nipposinica, and having a biofilm-formation suppression action.本發明係提供與已存在的抗菌劑等比較，具有耐性菌出現風險低等優點之嶄新觀點之牙周病預防劑。含有水菜冷水萃取物之具有生物薄膜形成抑制作用之口腔用組成物。
