The present invention provides a method of manufacturing packaged completely cooked rice or a packaged completely cooked mixed rice, comprising a steaming process of sterilizing and primarily cooking the contents in the opened container by filling the controlled steam having high pressure and high temperature in the steaming unit a cooking process of additionally sterilizing the contents in the opened container by filling the controlled steam having high pressure and high temperature in the steaming unit, and supplying water in the contents, and then completely cooking the contents and a sealing process of supplying clean nitrogen gas to the completely cooked contents in the sealing unit and sealing the upper end of the opened container with the lid film, wherein the steaming unit, cooking unit and the sealing unit are connected in turn in the one container so that the opened container can be input into and output from each of the units. Therefore, the contents selectively combined from various cereal crops including rice, nuts and other additives including powders of the cereal crops, the nuts or the like are provided in a completely cooked state so that a user can eat instantly without a separate heating process.
