A welding device for automatically welding a workpiece by a welding robot using a welding wire includes a welding control device that controls operation and welding work of the welding robot. The welding control device includes a sensing unit configured to detect a position of the workpiece, a root gap calculating unit configured to determine a root gap, and a storage unit including wire melting information as a database of a proper welding current corresponding to a feeding rate for each of the welding wire. A lamination pattern and a welding condition are provided in accordance with the root gap determined by the root gap calculating unit and the wire melting information so that an amount of heat input is equal to or less than a predetermined amount of heat input.