Although pure Ag has a large casting porosity near the sprue attachment, Ag-Zn and Ag-Cu alloys have small or no porosities. The casting porosities of Ag-Au alloys are similar to that of pure Ag. These experimental results were discussed using the calculated oxygen contents in molten state and the measured oxygen contents of castings, and related to the capacity for deoxidization of additive elements. In the present study the casting porosities in pure Ag and Ag-Au alloys are considered to be not shrinkage porosities, but porosities due to discharged oxygen gas. In spite of high oxygen contents of molten state and castings, the casting porosity was not observed in pure Cu. This may be due to the fact that the occluded oxygen atoms exist as copper oxide (Cu2O) in solid Cu.