A treatment instrument includes: a first holding surface including a first high frequency electrode portion a second holding surface configured to hold living tissue cooperatively with the first holding surface and including a second high frequency electrode portion a blade configured to apply pressure to the living tissue held between the first and second holding surfaces by a projected surface and a heater which is capable of heating the first holding surface to a second temperature higher than the first temperature and sufficient to incise the living tissue held between the first and second holding surfaces 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.
