A treatment instrument includes a first clamping surface, a second clamping surface configured to cooperate with the first clamping surface and to clamp a living body tissue, a pressure application member provided on one of the first and second clamping surfaces, and configured to apply a pressure to the living body tissue in a line shape by causing the first and second clamping surfaces to relatively approach each other, and a moving mechanism provided on the first clamping surface, and configured to move the first clamping surface in a manner to apply tensile forces to the living body tissue, clamped between the first and second clamping surfaces, in directions deviating away from a direction along a line formed on the living body tissue by the pressure application member.