A treatment instrument includes a treatment body that includes a treatment surface provided in the first clamp surface, the treatment surface protruding as much as a first protrusion amount from the first clamp surface when the supply of heat energy is stopped and protruding a second protrusion amount protruding more than the first protrusion amount from the first clamp surface in response to the supply of the heat energy, in a state where the living tissue is clamped between the first and second clamp surfaces, the treatment surface increasing pressure on the living tissue when reaching the second protrusion amount from the first protrusion amount, to treat the living tissue in cooperation with the action of the heat energy.
