A prototype tool tying a clove hitch to bind a wire harness is developed. Wire harnesses installed in airplanes are bound with belt-like strings. The binding process, tying a clove hitch first and then a double overhand knot, is still performed by human workers. To promote the efficiency of the process, simplification of the process with some tools is more practical than automation by robots. We propose a tool with a groove to form a clove hitch by getting a string through it. The bottom shape of the groove is very important to get the belt-like string through the groove without jamming. We formulated its shape based on the theory of surfaces and designed the shape by minimizing change in its curvature and torsion. We made a prototype tool and confirmed its performance with tying experiments. As a result, tying succeeded with a probability of 72%.