PURPOSE: To obtain a defectless weld zone which is stable even under low heat input conditions by converting a droplet droplet transfer state to a spray.

CONSTITUTION: The inter-electrode distance in AC submerged arc welding in which a preceding electrode and succeeding electrode are longitudinally arranged in the welding advance direction is made 5W20mm and the phase difference between both electrodes is made 60° or 90°. The wire diameter of the respective electrodes is made 1.2W3mm and the welding heat quantity H (KJ/cm) corresponding to a groove gap width G is set for the narrow groove joint having ≥8mm groove gap width G under the condition of 60W250A/mm2 current density. More specifically, melti-layer building-up by the single layer single pass satisfying equation I in the case of ≤18mm G and the single layer two pass lamination satisfying the equation II in the case of >14mm G is executed. The narrow groove 2-electrode submerged arc welding is thus executed.