Quality of a multi-layer welding depends on a welding in first layer. In order to get a high quality of the welding, it is necessary to make a stable back bead and to melt the base metals. In narrow gap welding, it is difficult to control the bead height (deposited metal) and the back bead (the melted area of the backside) simultaneously by using conventional welding. To improve this problem, authors propose a cooperative control of the welding equipments including the welding power source, the electrode wire feeder and the welding robot, i.e., the welding torch is not only oscillating on the groove, but also moving back and forth. To control the heat input, the power source is controlled according to the torch motion (robot motion), too. In the forward process, the arc (the heat) is given to the root edges to get a wide back bead. In the backward process, the suitable bead height is obtained. The idea of its robot motion comes from the knowledge of the skilled welding workers. Since the arc length must be kept short during the welding, the power source with a nonlinear power source characteristic is used. Welding conditions are investigated for 4 mm root gap, 5 mm root gap and 6 mm root gap by observing arc position with a high speed motion video camera, and observing the weld pool with a CCD camera. The welding conditions are stored in a database. Before the welding, the root gap at the beginning and at the end on the base metal is detected by a slit laser light and the CCD camera, and is stored into a memory in the personal computer. During the welding, the personal computer detects the torch position from the encoder pulses of the welding robot, finds the root gap at the torch position and calculates the welding conditions by using the data base, i.e., the adaptive control for the gap variation is applied. The welding experiment under the root gap variation from 6 mm to 4 mm is carried out to verify the validity of the cooperative control method.