PURPOSE: To shorten a welding time even in case of a narrow beveling and to obtain a good bead shape and mechanical performance by limiting the contents of more than the one kind of SiO2, TiO2, Al2O3, MnO, Caf2 CaO, ZrO2 and BaO, MgO and further more than one kind of Li2O, Na2O and K2O, and the grain size blending.

CONSTITUTION: The titled flux contains by weight 8W20% SiO2, 17W27% TiO2, 18W35% Al2O3, 3W10% Mn, 11W20% CaF2, 9.5W18% CaO, 2W7% ZrO2 and Al2O3/TiO1=0.80W1.55, SiO2+MnO+ZrO2≤25%. It further contains 2W5% the total of more than one kind of BaO and MgO and 1.5W3.5% the total of more than one kind of Li2O, Na2O and K2O and has the following grain size blending. Namely, the median diameter of the grain occupying 50% in the flux grain size distribution is 100W175μm and the grain size blending is in the range of the coarser grain than the 295μm of the flux grain diameter being less than 4% and the finer grain than 74μm 10W25%.