A clamp device includes a support mechanism that supports a grip member when expanding the diameter thereof, a clamping hydraulic cylinder that can drive the grip member and a clamp rod axially forwards and backwards (up and down), and a main body member to which the grip member, the clamp rod, the support mechanism, and the clamping hydraulic cylinder are mounted, with the grip member, the clamp rod, and the support mechanism being positioned at one end portion of the body member, and with the common axis of the grip member and the clamp rod being offset from the axis of the clamping hydraulic cylinder in a direction orthogonal to the clamp rod.