Abstract: An apparatus is disclosed for the internal welding of an edge joint between a large main pipe and a small branch pipe, which pipes together form a pipe branch. The apparatus is equipped with a nozzle holder for a non-consumable electrode which forms an acute angle with the axis of the branch pipe, said nozzle holder being introduced into the branch pipe and being capable of axial displacement by means of an actuating mechanism. The nozzle holder is connected to a support holder for a frame attached to the branch pipe. The support holder and the nozzle holder are capable of rotation about the branch pipe. The radial gap between the electrode and the branch pipe is determined by the interaction with a sensor which follows the wall of the pipe. The axial gap between the electrode and the edge joint, that is to say the point of welding, is adjusted by means of a comparison between the arc voltage corresponding to the actual gap and a desired gap arc voltage.