Abstract:This paper introduces the application of a robot to CO2gas shielded arc welding in the construction of steel structures. In shop welding, using horizontal position welding makes the construction process easier and more efficient. Technically, however, horizontal position welding is more difficult than the flat position one. So we have developed a horizontal position multilayer welding system by introducing a robot to the CO2gas shielded arc welding of steel structures where semiautomatic welding has been used. The situation where we can use this welding robot is in a single beveled horizontal position butt weld in a steel structure. In this paper, first we explain the structure of a welding robot system, followed by the results of a mechanical property test consisting of a tensile, impact test (based on the Japan Industrial Standard) and a full‐size three‐point bending test. These results were as good as those for semiautomatic welding. Using this horizontal position multilayer welding robot system, the burden of the welding operator will be reduced, and we will be able to obtain more easily constant high‐performance horizontal position we