To provide a container mouth sealing method which makes it conspicuous that a container is opened by conspicuously showing the perforation of a seal covering a container mouth being cut off.

In sealing a container mouth, the cap 12 of the container mouth and the neighborhood of the container mouth are airtightly covered using a shrink film 10, the film 10 is formed with a main cut off perforation 22 going around the container mouth and at a predetermined distance from the perforation, a plurality of auxiliary cut off perforations 24 are formed at several spots in a predetermined length in the direction going around the mouth. One end of each perforation 24 is formed with an end cut off perforation 26L (or 26R) going toward the perforation 22 roughly in a right angle direction.