To provide a compact and inexpensive bag opener wherein a mouth of an air permeable bag can be automatically and reliably opened.

The bag opener 1 includes a first holding plate 3 and a second holding plate 4 which are opposite across a bag mouth 2 so that they can come closer to/farther away from each other and a third holding plate 5 and a fourth holding plate 6 which are provided above the first holding plate 3 and the second holding plate 4 respectively, which come closer to/farther away as the first and second holding plates 3, 4 do and which are opposite across the bag mouth 2, wherein faces of the first to fourth holding plates 3, 4, 5, 6 in contact with the bag mouth 2 are anti-slipping faces 9, 10 and the first holding plate 3 and the third holding plate 5 can come closer to/farther away from each other while the second holding plate 4 and the fourth holding plate 6 can come closer to/farther away from each other.