Abstract: Herein disclosed is an automatic welding apparatus which comprises an arcuate frame including a welding unit and a pair of wire feeders, and an arcuate frame including a running truck and a pair of clamping mechanisms for fastening the outer circumference of a pipe to be welded. The aforementioned two arcuate frames are detachably connected at two positions thereby to form a ring shape. Thus, the pipe is automatically welded while the truck is being made to run in the circumferential direction. The running truck of the welding apparatus thus constructed carries both a mechanism for effecting close contact with the outer circumference of the pipe and a shock absorbing mechanism so that the distance between the leading end of a welding torch and the outer circumference of the pipe may be constant at all times.