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 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 (132, 172) 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 higher than the first temperature 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 (134, 174); 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 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 (132, 172) 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 higher than the first temperature 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 (134, 174); 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.