PURPOSE: To elongate the preservation period of the liquid food packed in a packaging container by fluid-tightly sealing the liqueform food in a flexible container and applying predetermined pressure to the same for a predetermined time.

CONSTITUTION: A container 5 is formed by a flexible outer casing wrapping a liqueform food, and sealed to prevent bacteria from intruding therein. The liquid 18 is filled between a cylinder wall and the container 5 in a cylinder space 3. When the container 5 is inserted into the cylinder space 3, a ram portion 2 is pressed down into the cylinder space 3 of a base 1, whereby the liquid 18 and the container 5 are pressurized, and the pressure of the liqueform food in the container 5 is raised. When the pressure in the container 5 is raised, the killing process of the microbial flora in the liqueform food is started. When the pressure is raised over 1000 (atm), the killing process is accelerated. On this occasion, the pressurizing time is determined dependently on the property of the liqueform food, a content of bacteria and the like, but continuously during a time of less than 30 minutes.