The　lamellar　structure　of　tungsten　inert　gas　(TIG)　arc　is　distinct　in　that　it　reflects　its　inhomogeneous　energy　distribution.　In　terms　of　an　arc＇s　physical　properties,　the　lamellar　structure　may　be　divided　into　three　regions;　the　ionization　region,　the　recombination　region　and　gas　fringes.　A　large　gradient　of　electron　density　exists　between　regions;　this　causes　differences　in　the　physical　properties　of　an　arc.　In　this　study,　the　scale　of　ionizing　regions　in　TIG　arc　is　accurately　determined　with　experimental　conditions　by　a　spectroscopic　diagnostic　method,　which　is　also　used　to　investigate　the　distribution　of　the　electron　density　of　the　arc.　More　precise　dimensions　of　arc　energy　distribution　are　described;　the　paper　also　discusses　the　difference　with　or　without　the　influence　of　metal　vapor　from　the　weld　pool.