A treatment instrument 3 of the invention comprises an elongated treatment instrument body a nozzle portion 23 provided at the distal side of the treatment instrument body for injecting a liquid W flowing to the inside of the treatment instrument body from the distal end of the treatment instrument body in the atomized state and an electrode 20 provided at a distal side relative to the nozzle portion 23 for discharging current supplied from a high-frequency power source transmitted from the proximal side to the distal side of the treatment instrument body through an electric conductive member along the atomized-state conductive fluid injected from the nozzle portion 23.
