Currently, the number of construction cases using large-diameter and high-strength steel and high-strength concrete is increasing due to the trend of large buildings. In the case of reinforcing bars that serve as the framework of a structure, the continuous state is the best in terms of structural stability. However, for convenience, it is transported and assembled to a predetermined standard. In this study, a coupler was developed applying SCM440 material with excellent mechanical properties, not S35C and S45C materials, generally used as coupler materials. To this end, high-frequency carburizing and heat treatment was applied to the element parts taking into account the taper angle and stress results, reflecting the results of low- and high-cycle fatigue tests and structural analysis for the applied material. Finally, in the case of a reinforced coupler fastened with hydraulic SD500 reinforcement bars with diameter D25, a reliability test was carried out using the mechanical joint inspection method of reinforced concrete reinforcement bars. Results were obtained that satisfied the characteristic performance values.