PURPOSE: To perform welding uniformly and improve weld quality by changing the welding speed in corresponding to the grade of a curved slope in arc welding for curved slope.

CONSTITUTION: In the case of maintaining welding electric current and are voltage constant, and performing welding by advancing a torch from the vertex of a crest shape curved slope to right and left down directions, the welding speed is made high as the grade of the arc is actuter. Hence, the welding speed of a steel segment having small diameter is accelerated fallowing up the advancement of the welding torch to the C, C' line. Conversely, the welding speed is made lower as the grade of the arc is gentler. Thus, the welding is accomplished at the welding speed proportional to the fluidity of the molten metal, whereby the dropping of the molten metal and the enlargement of bead width are prevented.