A treatment device includes a first grip member having an elongated shape, and a second grip member 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. The second grip member includes a grip section that comes into contact with a biological tissue as a gripping target. 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.