A welding parameter derivation device of a welding machine having a torch and a weaving mechanism derives welding parameters in accordance with the cross-sectional shape of a weld portion of a new base metal. A database stores welding parameter data, and a welding parameter computation unit computes welding parameters for the shape of a groove or joint of a new base metal. Based on past welding parameter data for a shape similar to that of a groove or joint of a new base metal, and input data pertaining to the specifications of the welding machine, the computation unit derives welding parameter data for the new base metal, taking into account a parameter of the cross-sectional area of the weld portion formed on the new base metal, the bead height of the weld portion, the quantity of heat inputted to the new base metal, and a torch weaving parameter.