An ultrasonic treatment device comprising: a sheath which includes a sheath outer peripheral portion and a sheath inner peripheral portion, and which is extended along a longitudinal axis; a probe which includes a probe distal surface and a probe outer peripheral portion, and which is inserted through the sheath in a state that the probe distal surface is positioned to a distal direction side of a distal end of the sheath; a suction passage defining portion which defines a suction passage along the longitudinal axis inside the probe; and a clearance defining portion which defines a clearance between the sheath inner peripheral portion and the probe outer peripheral portion; a suction port defining portion which defines at least one suction port (28A, 28B) in the probe outer peripheral portion, each suction port (28A, 28B) communicating with the suction passage and an opening defining portion which defines at least one opening (58A, 58B; 58A) in the sheath outer peripheral portion, each opening (58A, 58B; 58A) communicating with the clearance, characterised in that; each suction port (28A, 28B) has a non-exposed portion whose outer peripheral direction side is covered with the sheath, and an exposed portion whose outer peripheral direction side is uncovered by the sheath; and in that a gas being configured to flow into the clearance from an outside of the sheath through each of the at least one opening (58A, 58B; 58A) and then the gas flowed into the clearance being configured to flow into the suction passage through the non-exposed portion of each suction port (28A, 28B) when the suction force is caused in the suction passage. a sheath which includes a sheath outer peripheral portion and a sheath inner peripheral portion, and which is extended along a longitudinal axis; a probe which includes a probe distal surface and a probe outer peripheral portion, and which is inserted through the sheath in a state that the probe distal surface is positioned to a distal direction side of a distal end of the sheath; a suction passage defining portion which defines a suction passage along the longitudinal axis inside the probe; and a clearance defining portion which defines a clearance between the sheath inner peripheral portion and the probe outer peripheral portion; a suction port defining portion which defines at least one suction port (28A, 28B) in the probe outer peripheral portion, each suction port (28A, 28B) communicating with the suction passage and an opening defining portion which defines at least one opening (58A, 58B; 58A) in the sheath outer peripheral portion, each opening (58A, 58B; 58A) communicating with the clearance, characterised in that; each suction port (28A, 28B) has a non-exposed portion whose outer peripheral direction side is covered with the sheath, and an exposed portion whose outer peripheral direction side is uncovered by the sheath; and in that a gas being configured to flow into the clearance from an outside of the sheath through each of the at least one opening (58A, 58B; 58A) and then the gas flowed into the clearance being configured to flow into the suction passage through the non-exposed portion of each suction port (28A, 28B) when the suction force is caused in the suction passage.