We developed the wire antagonistic type robot finger using nonlinear torque joint mechanism. This robot finger has extension wire and flexion wire. The posture of the finger is decided uniquely by the relation between extension torque which is generated by the nonlinear torque joint mechanism, flexing torque which is generated by tension given to a flexion wire, and additional extension torque which is generated by tension given to a extension wire. This robot finger can be switched adaptive grasp movement and pinching movement by adjusted the tension of the extension wire. In this paper, we derived the conditional expression of the torque balance for designing the robot finger. The validity of this expression and be able to switch the two behaviors were confirmed experimentally.