Tungsten Inert Gas (TIG) welding with a constricted nozzle is one of new higher-level welding processes. In this process, an additional gas nozzle which is called ``constricted nozzle'' is attached between a tungsten electrode and a normal gas nozzle. This process achieves a welding of quite thin sheet metal whereas it is di‹cult for the conventional TIG welding process. In this study, numerical simulations with a focus on the conventional TIG welding and the TIG welding with a constricted nozzle were performed in order to understand the arc phenomena and clarify the effect of the constricted nozzle. The current density was concentrated to the center part of the arc plasma because the high speed gas which flows along the electrode cools the edge of arc plasma. As a result, the velocity of the plasma jet was greatly increased and the temperature of the arc plasma increased more than 2000 K in the TIG welding with a constricted nozzle. These characteristics make the arc plasma stable and high energy density and contribute to the welding of quite thin sheet metal.