A treatment instrument configured to treat a living tissue by applying energy thereto includes: first and second holding members; a swing member supported by the first holding member and being rotatable in a first direction and in a second direction opposite to the first direction on a rotation axis as a supporting point; a limiting portion which limits the rotation of the swing member in one of the first direction and the second direction; a first holding surface provided on the side of the swing member close to the second holding member and configured to hold the living tissue; a second holding surface provided on the side of the second holding member close to the swing member and cooperating with the first holding surface of the swing member to hold the living tissue; and a protrusion forming a clearance between the first and second holding surfaces when the first and second holding surfaces are closed relative to each other.