Due to unbalanced phases' contents in welds of duplex stainless steel, their mechanical properties, and corrosion resistance should be remarkably degraded from base metals' ones. It was reported that both the precipitation and growth of the austenite phase in the welds are affected by the nitrogen contents as well as thermal cycles of welding. Especially, in the case of laser beam welding (LBW), these effects might be drastically changed as compared with them in conventional welding methods. From the microstructure observation in the LB welds used Argon shielding gases varied with nitrogen contents of 0%, 10%, and 20%, austenite phases growth at grain boundaries were promoted by the increase in nitrogen contents in the shielding gases. Based on elements' distributions measured in the previous report by an EPMA analysis, nitrogen was powerfully concentrated in austenite in each case. In addition, the in-situ observation by the micro-focused X-ray transmission imaging system revealed that porosity in the LB welds was suppressed by the increase of nitrogen in Argon shielding gases. That is, these results suggested that under suitable thermal history conditions, the addition of nitrogen in Argon shielding gas for LBW might be effective to increase in austenite contents in welds of duplex stainless steels.