A link mechanism for an arm portion includes an arm portion, a non-rotating arm attachment portion, a variable-length rod, and a fixed-length rod. The arm portion holds a medical instrument. The arm portion is attached to the arm attachment portion to be rotatable about an arm rotational axis as a center. The variable-length rod includes an elastic member which biases an elastic force to bring about a recovery against a pressing force. One end of the variable-length rod is pivotably attached to the arm attachment portion and the other end of the variable-length rod is pivotably connected to one end of the fixed-length rod. The fixed-length rod has its other end attached to the arm portion to be movable about the arm rotational axis upon rotation of the arm portion. When the center of gravity of the arm portion is horizontally aligned with the arm rotational axis, the direction of action of the elastic force is set so as to be matched with a tangential direction to the rotational trajectory along which the other end of the fixed-length rod is movable.
