The authors previously reported about the harmful light generated by metal inert gas welding (MIG) of aluminum alloys and magnesium alloys. At that time, the authors suggested that the fume may affect the intensity of harmful light. In the present work, the amount of fume and particle size of the fume for the MIG welding of both types of alloys are studied, and a qualitative analysis is conducted for both cases. The following observations were made: (1) In case of aluminum alloys, the amount of magnesium in the welding wire has a stronger influence on the amount of fume than that of the base metal, and the fume amount is almost proportional to the welding current. (2) The amount of fume for the magnesium alloy is approximately thrice that for the aluminum alloy. (3) With regard the particle size of fumes generated in both alloys, the amount of particles of size 0.33-0.76 μm that are trapped is approximately 70％of the total. The amount of particles of size 5.0 μm or less, which have a large influence on the human body, is approximately 95％. (4) It is inferred that the main component of fumes is Al2O3 and MgAl2O4 for the aluminum alloy, and MgO for the magnesium alloy.