A treatment device includes a treatment portion, a transmission portion and a moment shift portion. The treatment portion transmits ultrasonic vibration. The length of the treatment portion is shorter than ¼ of a wavelength of the ultrasonic vibration. The transmission portion has one end at a node position where the first generated node from the distal end of the treatment portion appears, and is located closer to a proximal side than the node position. The length of the transmission portion is ¼ of the wavelength. The moment shift portion is located between the treatment portion and the transmission portion. An average of I1/A12 is greater than an average of I2/A22, where I1 is the second moment of area and A1 is a sectional area in the treatment portion, and I2 is the second moment of area and A2 is a sectional area in the transmission portion.
