In an oxidizing gas welding atmosphere of Ar-O2, Ar-CO2 or CO2-O2 gas mixture, oxygen absorption behavior by steel weld metal has been investigated metallurgically, using a popular Fe-Si-Mn electrode wire and two types of base metal, that is, pure iron and rolled steel for welded structure. As an addition of oxygen to argon-based mixture, the oxygen content in steel weld metal extremely increases and the losses of alloying elements such as silicon and manganese steadily increase. As an addition of carbon-dioxide to argon-based mixture, the oxygen content in steel weld metal increases somewhat and limited amount of alloying elements losses. As an addition of oxygen to carbon-dioxide-based mixture, only a little amount of oxygen is absorbed into the steel weld metal. Despite the active gas of CO2 instead of Ar adds into the welding atmosphere, unexpectedly, the oxygen content of steel weld metal under CO2-O2 is lower than that of the weld metal under Ar-O2 for the same oxygen partial pressure over the range of 0.01 MPa. The losses of alloying elements are not so significant in this atmosphere.