In the previous study, we applied humid gas stress corrosion cracking (HG-SCC) test to metal inert gas (MIG) welded 5083 plate with an attention to the location of the pre-crack, and revealed that stress corrosion cracking (SCC) based on hydrogen embrittlement (HE) only progressed at weld metal region adjacent to base metal (weld corner). In the present study, we have investigated the cause of the SCC of the MIG-welded 5083 aluminum plate in humid air, in relation between local Mg accumulation, microstructure and crack propagation. The results of scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) with the aid of optical microscope (OM) observation showed coarse Al₃Mg₂ or Al₈Mg_{5 β} phase particles on grain boundaries in the weld corner region, in accord with the microscopic Mg content profile, which must be the result of the segregation during solidification combined with columnar grain growth. The average Mg content in this region was significantly larger than in the other weld and base metal regions. From these facts, together with the reported increase in solute Mg concentration with increasing distance from the weld center, it was deduced that fine film-like precipitation of β phase in the weld corner is prone to occur only in this region at sensitization temperatures, during solidification which must be the main cause for the low resistance to HG-SCC.