An inert gas-filled cooking process is disclosed which provides for quick sterilization of food in inert gas-filled packages in a manner that prevents deterioration of food quality as well as assures a long taste guaranteed period by raising the temperature of hot water to be applied onto the food packages in a sterilization tank so as to satisfy the retort sterilization requirements. The sterilization tank is pressurized at a predetermined value by control of an analog pressure regulating valve. The temperature of the hot water flowing from a heat exchanger into the sterilization tank is varied in a multi-staged fashion to follow a predetermined sterilization temperature-time curve by control of an analog steam regulating valve and a steam superheater. The hot water is applied onto the packaged food for sterilization.