A treatment method is provided that is entirely different from conventional treatment methods in that it is capable of sufficiently suppressing pain and the recurrence of all the bacterial intraoral diseases without depending on removal of living tissues. This treatment method is a lesion-sterilizing and tissue-repairing treatment method including the processes of: washing teeth affected with bacterial intraoral disease with a washing treatment solution; administering a treatment composition to the washed teeth; and covering an opening of the teeth. This treatment composition includes an antibacterial agent having an antibacterial property against intraoral bacteria and a base containing polyethylene glycol 400, polyethylene glycol 600, polyethylene glycol 6000, and propylene glycol.