Abstract: A vertical down welding process by gravity arc welding, in which a guide rail 5a is disposed aslant upwardly at an inclination angle .theta..sub.1 of 40.degree. to 60.degree. to a vertical weld line 3 and, while a holder 6 of a welding rod 4 is brought down along the guide rail 5a, the welding rod 4 is moved down along the vertical weld line 3 for welding.A welding machine for use with this vertical down welding process is constructed so that the upwardly inclined guide rail 5a is provided at the inclination angle .theta..sub.1 of 40.degree. to 60.degree.;, the guide rail 5a is comprised of a main rail 5c and a sub-rail 5b; and the holder 6 of the welding rod 4 is lowered from the sub-rail 5b to the mail rail 5c.