A treatment device comprising: a treatment portion which has a first distal end portion and a first proximal end portion and which transmits ultrasonic vibration generated in an ultrasonic vibrator from the first proximal end portion to the first distal end portion in a longitudinal direction, a length of the treatment portion from the first distal end portion in the longitudinal direction being a first length, and the treatment portion being used for grasping a living tissue; a transmission portion having a second distal end portion, the second distal end portion being located at a node position where the node position is a position where a first generated node from the first distal end portion of the treatment portion appears, a second length of the transmission portion in the longitudinal direction being 1/4 of a wavelength of the ultrasonic vibration, and the transmission portion comprising a piezoelectric element as the ultrasonic vibrator; and a moment shift portion located between the treatment portion and the transmission portion, a length of the moment shift portion in the longitudinal direction being a third length, and a grasp member which grasps the living tissue with the treatment portion, wherein an average value of I 1/ A 1 2 of each point on the treatment portion is greater than an average value of I 2 /A 2 2 of each point on the transmission portion, where I 1 is a second moment of area in the treatment portion calculated with respect to an axis perpendicular to a direction to which a load is applied by grasping the living tissue in a section perpendicular to the longitudinal direction, A 1 is a sectional area in the treatment portion, I 2 is a second moment of area in the transmission portion, and A 2 is a sectional area in the transmission portion, a total length of the first length in the treatment portion and the third length in the moment shift portion is 1/4 of the wavelength of the ultrasonic vibration, and the treatment portion and the moment shift portion each have a U-shaped section perpendicular to the longitudinal direction. a treatment portion which has a first distal end portion and a first proximal end portion and which transmits ultrasonic vibration generated in an ultrasonic vibrator from the first proximal end portion to the first distal end portion in a longitudinal direction, a length of the treatment portion from the first distal end portion in the longitudinal direction being a first length, and the treatment portion being used for grasping a living tissue; a transmission portion having a second distal end portion, the second distal end portion being located at a node position where the node position is a position where a first generated node from the first distal end portion of the treatment portion appears, a second length of the transmission portion in the longitudinal direction being 1/4 of a wavelength of the ultrasonic vibration, and the transmission portion comprising a piezoelectric element as the ultrasonic vibrator; and a moment shift portion located between the treatment portion and the transmission portion, a length of the moment shift portion in the longitudinal direction being a third length, and a grasp member which grasps the living tissue with the treatment portion, wherein an average value of I 1/ A 1 2 of each point on the treatment portion is greater than an average value of I 2 /A 2 2 of each point on the transmission portion, where I 1 is a second moment of area in the treatment portion calculated with respect to an axis perpendicular to a direction to which a load is applied by grasping the living tissue in a section perpendicular to the longitudinal direction, A 1 is a sectional area in the treatment portion, I 2 is a second moment of area in the transmission portion, and A 2 is a sectional area in the transmission portion, a total length of the first length in the treatment portion and the third length in the moment shift portion is 1/4 of the wavelength of the ultrasonic vibration, and the treatment portion and the moment shift portion each have a U-shaped section perpendicular to the longitudinal direction.