A treatment instrument comprising: a first holding surface which includes a first high frequency electrode portion ; a pair of second high frequency electrode portions which includes a second holding surface opposed to the first holding surface , configured to hold living tissue cooperatively with the first holding surface , configured to supply high frequency energy to the living tissue, and configured to raise a temperature of the living tissue held between the first high frequency electrode portion and the second high frequency electrode portion to a first temperature (T1) sufficient at least to denature protein, to seal the living tissue by heat cooperatively with the first high frequency electrode portion ; a blade which is provided on the first holding surface , which includes a projected surface projecting from the first holding surfaces , which is formed at a center of the first holding surface in a width direction (β), and which is configured to apply pressure to the living tissue held between the first and second holding surfaces by the projected surface ; and a heater which is configured to generate heat and supply the heat to the first high frequency electrode portion , wherein the heater is capable of heating the first holding surface to a second temperature (T2) higher than the first temperature (T1) and sufficient to incise the living tissue by the blade , upon receipt of energy supplied at the same time as or after energy supply to the first and second high frequency electrode portions ; wherein the second holding surface includes a receiving portion having electrical insulation properties, and is brought into contact with the projected surface of the blade ; and wherein the receiving portion has a concave shape, a plane or smooth curved surface which is provided between the pair of second high frequency electrode portions . a first holding surface which includes a first high frequency electrode portion ; a pair of second high frequency electrode portions which includes a second holding surface opposed to the first holding surface , configured to hold living tissue cooperatively with the first holding surface , configured to supply high frequency energy to the living tissue, and configured to raise a temperature of the living tissue held between the first high frequency electrode portion and the second high frequency electrode portion to a first temperature (T1) sufficient at least to denature protein, to seal the living tissue by heat cooperatively with the first high frequency electrode portion ; a blade which is provided on the first holding surface , which includes a projected surface projecting from the first holding surfaces , which is formed at a center of the first holding surface in a width direction (β), and which is configured to apply pressure to the living tissue held between the first and second holding surfaces by the projected surface ; and a heater which is configured to generate heat and supply the heat to the first high frequency electrode portion , wherein the heater is capable of heating the first holding surface to a second temperature (T2) higher than the first temperature (T1) and sufficient to incise the living tissue by the blade , upon receipt of energy supplied at the same time as or after energy supply to the first and second high frequency electrode portions ; wherein the second holding surface includes a receiving portion having electrical insulation properties, and is brought into contact with the projected surface of the blade ; and wherein the receiving portion has a concave shape, a plane or smooth curved surface which is provided between the pair of second high frequency electrode portions . <EOS>