An ultrasonic treatment device comprises a handpiece having an ultrasonic transducer and a long axial probe whose base end is connected with the transducer. The device further comprises a tubular inner sheath covering the probe and an outer sheath covering the inner sheath with a predetermined annular space left therebetween. The space between the sheaths is used as a suction path for mist. For the suction, the device further comprises a base-end sealing member made of an elastic material, disposed on a base-end side of the outer sheath and engaged detachably with the inner sheath to seal a gap formed with the inner sheath. A suction unit is disposed on the base-end side of the outer sheath. The suction unit sucks in mist to be generated when ultrasonic treatment is performed, through the suction path, and discharged outside.
