In order to decrease the amount of air in a polystyrene foam pattern, we replaced the gas in the pattern with argon, and then measured the pattern decomposition rate and mold filling rate in the evaporative pattern casting of molten cast iron. The gas substitution in the pattern was performed by keeping the pattern in an argon atmosphere after the pattern was placed in a vacuum vessel under reduced pressure. The amount of air in the pattern after gas substitution was found to decrease with increasing holding times at reduced pressure and in an argon atmosphere. The pattern decomposition rate immediately after pouring tended to increase with increasing holding times at reduced pressure and in the argon atmosphere, however the decomposition rate slowed down and the thermal decomposition gas layer thickness increased with increasing time after pouring. In the initial stage of the mold filling, the mold filling rate tended to be faster with increasing argon atmosphere holding time during gas substitution, and slower with longer flow length of the molten metal. The decomposition rate of the pattern was slower when the pattern was coated after gas substitution than when the pattern was coated before gas substitution, suggesting that gas substitution efficiency is higher without coatings.