The formation of stable back beads in the first layer weld during one side multilayer welding is important to achieve high quality welded metal joints. The authors thus employed the switch back welding method for the welding of V groove joints without backing plates. In this method, the personal computers control a welding robot, a digital welding power source and a wire feeder, simultaneously. Each unit is connected together with Ethernet and UDP (User Datagram Protocol). The personal computer for controlling the welding robot is synchronized with other computers. The suitability of the welding conditions for each root gap was verified by observation of the arc, molten pool and external appearance of back beads.