PURPOSE: To obtain a beautiful appearance and excellent processability by a method wherein a film after treatment within an air oven possesses specific lateral and longitudinal shrinkage factors and lateral tearing strength.

CONSTITUTION: A shrinkage factor of a film after treatment within an air oven having a temperature of 100°C is at least 40% in a lateral direction and not exceeding 20% in a longitudinal direction and lateral tearing strength of the film is at least 600g/mm, in a polyester shrinkable film for cap sealing. In this case, in a film whose lateral shrinkage factor is less than 40%, a shrinkage quantity is insufficient and in a film whose longitudinal shrinkage factor exceeds 20%, since a warp or a curl of an end part is generated at the time of shrinkage, an appearance of the cap seal becomes inferior and the same cannot be put to practical use. Furthermore, in the case where the lateral strength of the film is less than 600g/mm, since break of the film occurs frequently at the time of performance of stamping of a perforated line in the film or after that and productivity is deteriorated very much, it is inadequate.