To provide a method of forming perforations by a laser beam on a laser beam absorbing layer of a laminated film for a seal opening, on which layer the cut line is not much noticeable.

In a method where perforations are formed by a laser beam on a laser beam absorbing layer of a laminated film provided with a laser beam absorbing layer and a layer of thermally adhesive resin, the method of forming perforations for opening seal is characterized by a process in which the laminated film is irradiated by a laser beam while the laminated film is in contact with a cooled supporting body.