A treatment device according to the present invention includes: an elongated sheath having flexibility, the sheath being passed through an endoscope and introduced to an object to be treated; an incision knife section disposed approximately along an axial line of the sheath around an outer periphery at the distal end thereof, the incision knife section being used for incising tissue of the object to be treated; and a rotation-torque-transmitting section extending from a proximal end of the elongated sheath maneuvered by an endoscopist to an intermediate section of the incision knife section in the axial line direction, the rotation-torque-transmitting section rotating the sheath by transmitting rotation torque input at the proximal end of the sheath.