Method for producing freshness-keeping bag

A method of producing a freshness-keeping bag formed of a film, wherein the freshness-keeping bag has a band-shaped joining region in which two ends of the film are overlapped and joined together with a width of 5 mm to 15 mm, the joining region comprising:

• a plurality of joined parts in which the two ends of the film are joined directly together without any intermediary sheet between the two ends of the film;
• and
• a plurality of non-joined parts which are located between adjacent ones of the joined parts, wherein the overlapped ends of the film are not joined together in each non-joined part, each non-joined part comprising a non-linear ventilation passage devoid of any obstructing filling throughout the passage and going through the joined parts to establish communication between an internal space of the bag and an outside of the bag, and each ventilation passage having a width W and a length L satisfying the following relations:
• 0.2 mm≤W≤3 mm, and 10 mm≤L≤50 mm, respectively,
• the method comprising the step of forming the joined parts by ultrasonic welding, wherein the step of forming the joined parts includes continuously forming the joined parts in the film, while rotating a disk-shaped ultrasonic welding anvil with respect to the film, the ultrasonic welding anvil having a plurality of protrusions formed on an outer periphery thereof continuously at given intervals and correspondingly to a configuration of the joined parts of the bag.