A treatment device for use in treating a biological tissue, includes: an ultrasonic transducer unit a probe an actuator that moves the ultrasonic transducer unit and the probe integrally along the longitudinal axis at a stroke larger than an amplitude of the ultrasonic vibration to be transmitted to the probe an inner sheath that covers an outer peripheral surface of the probe and move with the probe accordance with a movement of the actuator a first sealing member that seals between an inner peripheral surface of the inner sheath and the outer peripheral surface of the probe and that prevent penetration of a liquid to a proximal side of an outer peripheral surface of the probe along the longitudinal axis from an seal position an outer sheath that is disposed on an outer peripheral surface of the inner sheath and a second sealing member that seals between the outer peripheral surface of the inner sheath and an inner peripheral surface of the outer sheath and that prevents the penetration of the liquid to an out peripheral surface of the probe and the ultrasonic transducer unit.
