A bag opening apparatus comprising a gap formation unit for bending at least a front wall portion, of the front wall portion and a back wall portion that constitute a mouth portion of a bag body, so as to form a gap between the front wall portion and the back wall portion, wherein: the gap formation unit includes a first holding unit and a first press drive member for moving the first holding unit between a position where the first holding unit is separated from the front wall portion and a position where the first holding unit is pressed against the front wall portion, and the first holding unit includes a contact part, a support part which swingably supports the contact part, and an urging part for urging the contact part via the support part, characterized in that the gap formation unit is arranged such that when the first press drive member presses the first holding unit against the front wall portion, the contact part is swung against urging force of the urging part while the contact part is in contact with the front wall portion so as to bend the front wall portion, the gap formation unit further includes a second holding unit, the first press drive member is adapted to move not only the first holding unit but also the second holding unit between the position where the first holding unit and the second holding unit are separated from the front wall portion and the position where the first holding unit and the second holding unit are pressed against the front wall portion, the gap formation unit is further arranged such that when the first press drive member presses the first holding unit and the second holding unit against the front wall portion, the contact part of the first holding unit is swung so as to approach the second holding unit while the contact part of the first holding unit is in contact with the front wall portion, the bag opening apparatus further comprises a pair of grippers for holding both side portions of the bag body, and the gap formation unit is further arranged such that before or while the first press drive member presses the first holding unit against the front wall portion it causes the contact part of the first holding unit to swing, at least one of the grippers being moved in such a manner that a distance between the grippers is reduced. the gap formation unit includes a first holding unit and a first press drive member for moving the first holding unit between a position where the first holding unit is separated from the front wall portion and a position where the first holding unit is pressed against the front wall portion, and the first holding unit includes a contact part, a support part which swingably supports the contact part, and an urging part for urging the contact part via the support part, characterized in that the gap formation unit is arranged such that when the first press drive member presses the first holding unit against the front wall portion, the contact part is swung against urging force of the urging part while the contact part is in contact with the front wall portion so as to bend the front wall portion, the gap formation unit further includes a second holding unit, the first press drive member is adapted to move not only the first holding unit but also the second holding unit between the position where the first holding unit and the second holding unit are separated from the front wall portion and the position where the first holding unit and the second holding unit are pressed against the front wall portion, the gap formation unit is further arranged such that when the first press drive member presses the first holding unit and the second holding unit against the front wall portion, the contact part of the first holding unit is swung so as to approach the second holding unit while the contact part of the first holding unit is in contact with the front wall portion, the bag opening apparatus further comprises a pair of grippers for holding both side portions of the bag body, and the gap formation unit is further arranged such that before or while the first press drive member presses the first holding unit against the front wall portion it causes the contact part of the first holding unit to swing, at least one of the grippers being moved in such a manner that a distance between the grippers is reduced.