PURPOSE: To prevent the occurrence of weld defects of the joint part root and to improve welding efficiency at the time of performing multilayer welding on extra-thick plates by submerged arc welding by using two electrodes by regulating Y-shaped groove shape dimensions and performing submerged arc welding of the root part under specified welding conditions.

CONSTITUTION: In multilayer welding of the extra-thick plates 1 and 1 by submerged arc welding by using the two electrodes of a preceding electrode L and a succeeding electrode T having 5.3-8.0mm diameter and a wire shape on a Y-shaped groove joint having 1-8mm root face (l) and 40-55° groove angle θ, the root part is subjected to submerged arc welding under the welding conditions of the distance (d) of 40-80mm between the electrodes, a wire tilt angle α of -15°-+5° at the preceding electrode L and 0°-+20° at the succeeding electrode T, wire extension length (h) of 40-120mm at the preceding electrode L and 50-120mm at the succeeding electrode T, 1000-2000A current of the L electrode and 800-1800A current of the T electrode, the current different of 100-600A between both electrodes L and T, the current ratio of 0.75-1.0 between both electrodes L and T and the welding speed of 15-45cm/min.