PROBLEM TO BE SOLVED: To provide a treatment instrument for preventing water from entering a joint part of living tissues and preferably maintaining the strength of the joint part, and to provide a treatment device and treatment method.SOLUTION: The treatment instrument for joining living tissues includes: at least one pair of holding members 52 for holding living tissues to be treated； an energy output part 92 provided on at least one of the pair of holding members and connected to an energy source for supplying energy to the living tissues held by the pair of holding members and connecting the living tissues to form a joint part； and a joint maintaining assistance part 92a that applies a substance for preventing water from entering living tissues to be treated in order to assist to maintain the joined state of the living tissues.
