Abstract: A high-efficiency fillet welding method for conducting, without edge preparation, T-shape fillet welding of thick steel webs having a C content not more than 0.21 wt %. The welding is conducted by using a leading electrode (L) and a trailing electrode (T) composed of wires of about 4.8 to 6.4 mm diameter and containing C in an amount which meets the condition of formula (1), as well as 1.50 to 2.50 wt % of Mn. The welding is executed in such a manner that the ratio I.sub.T /I.sub.L of the current I.sub.T supplied to the trailing electrode (T) to the current I.sub.L supplied to the leading electrode (L) ranges from 0.75 to 1.00. The flux used in the welding is a bonded flux containing 60 to 91 wt % in total of SiO.sub.2, MgO and CaO, 5 to 30 wt % in total of TiO.sub.2, Al.sub.2 O.sub.3 and CaF.sub.