PURPOSE: To prevent lateral cracking of penetration bead in welding of thin plate <16mm in thickness by forming the upper flux layer of the flux-copper backing method with fine grains mixed with solidifying agent and forming the lower layer with coarse grains and specifying total scattering thickness of the upper and lower layers.

CONSTITUTION: In scattering flux of 2,380W63μ in grain diamter on a copper backing 6 in two layers, ≥2mm of flux having average grain radius of 100W300μ is scattered to form a lower layer 5. Then, 2W10mm, in thickness, of flux having average grain radius of 55W90μ and containing 0.5W15wt% of material that is molten and solidified by heat of welding is scattered to form an upper layer 4. Total thickness of scattering of layers 4, 5 is made to ≥7mm. The layer 4 side of the backing 6 is placed on the back of the groove of steel plates 3 to be welded, and pressed by air hoses 7. One side shielded arc welding is performed by a wire 1 while scattering surface flux 2. Coherence to the plate 3 is improved and solidifying power of flux is increased by the layer 4, and gas can be discharged rapidly by the layer 5. Further, the distance between the plate 3 and the backing 6 is ensured, thereby the object is attained.