PURPOSE: To reduce dust which flies at the time of film processing and improve the working environment by a method wherein a part or the whole of a resin film is heated, and a notched groove for tearing is formed on the surface of a portion which is heated of the resin film.

CONSTITUTION: For a product such as a mirror-shaped rice-cake, etc., is airtightly packaged by a resin film 11, and a notched groove 5 for tearing is formed on said resin film 11 in order to make the opening easier, when the notched groove 5 is formed on the resin film 11, a leading end 14 of a horn part 13 of an ultrasonic oscillator 12 is brought into contact with the upper surface of the resin film 11 which is carried in the direction shown by arrow 10. Also, a roll 15 such as a heater roll equipped with a heating means is arranged in such a manner that the contact part of the leading end 14 of the horn part may be pressed from the lower side, on the lower surface of the resin film 11. Then, the notched groove 5 is formed on the resin film 11 by the horn part 13 while the resin film 11 is being heated by the roll 15. By this method, dust which flies when the notched groove is formed can greatly be reduced.