Treatment device

A treatment device comprising:

• a first grip member having an elongated shape;
• and
• a second grip member which comprises a grip section adapted to come into contact with a biological tissue as a gripping target and which is displaced with respect to the first grip member to grip the biological tissue between the first grip member and the second grip member, wherein an end of the grip section on a side connected with the first grip member is a proximal end;
• an end of the grip section on a free end side is a distal end;
• and I/A2 of the grip section is maximum at the proximal end, and/or Z/A3/2 of the grip section is maximum at the proximal end, where I is a second moment of area, Z is a section modulus, and A is a cross-sectional area calculated based on an axis perpendicular to a straight line passing through a center of gravity of the first grip member and a center of gravity of the second grip member in a cross section perpendicular to a longitudinal axis of the grip section, wherein
• when the first grip member and the second grip member grip the biological tissue, the second grip member is configured to ultrasonically vibrate, and
• a shape of the cross section of the grip section is an inverted U-shape in which a side opposite to a side facing the first grip member is open.