In carbon dioxide arc welding by use of shielding gas comprised of carbon dioxide as main component, a welding wire that enables spray transfer of globules, and provides an excellent bead shape in addition to reduction of amount of spatters even in a high-speed welding, and a welding method using the welding wire are proposed. As the specific means, a welding steel wire for use in carbon dioxide arc welding in DC-electrode negative, comprising a bar steel having a composition that contains 0.003 to 0.20% by mass of C, 0.05 to 2.5% by mass of Si, 0.25 to 3.5% by mass of Mn, 0.015 to 0.100% by mass of REM (rare-earth elements), 0.001 to 0.05% by mass of P, and 0.001 to 0.05% by mass of S, or further contains 0.0100% by mass or less of O, or further contains one or two or more of 0.02 to 0.50% by mass of Ti, 0.02 to 0.50% by mass of Zr, and 0.02 to 3.00% by mass of Al, or further contains 0.0001 to 0.0150% by mass of K, or further contains 3.0% by mass or less of Cr, 3.0% by mass or less of Ni, 1.5% by mass or less of Mo, 3.0% by mass or less of Cu, 0.015% by mass or less of B, 0.20% by mass or less of Mg, 0.5% by mass or less of Nb, 0.5% by mass or less of V, and 0.020% by mass or less of N, and contains Fe and unavoidable impurities as residue is used.