We propose a method of approximating a continuous curve by Active Cord Mechanism. In this method we only use deformation parameters of a curve such as curvature and torsion, and do not use coordinates of a curve, so the proposed method does not include solution of nonlinear equations and the calculation cost is less than former methods. We also discuss the convergence of approximating error in this method and show the error is inversely proportional to square of the number of links in planar case. The proposed method is simple and easy to apply, so it is useful for control of multi-joint robots, particularly for control of snake-like robots. We have implemented the proposed method on real snake-like robots and shown the merit of the proposed method by experiments. This result shows the proposed method is effective for study of snake-like robots.