A treatment device (10) includes a grasp member (170) and an ultrasonic vibration unit (140). The ultrasonic vibration unit (140) includes a treatment portion and a transmission portion. One end of the treatment portion is a distal end of the ultrasonic vibration unit, a length of the treatment portion being shorter than 1/4 of a wavelength of ultrasonic vibration. The transmission portion has one end at a first node position where a first generated node from the distal end of the ultrasonic vibration unit appears, the transmission portion being located closer to a proximal side than the first node position. An average of I 1 /A 1 2 is greater than an average of I 2 /A 2 2 , where I 1 is a second moment of area and A 1 is a sectional area in the treatment portion, and I 2 is a second moment of area and A 2 is a sectional area in the transmission portion.
