To provide a bag-in-box which prevents the formation of a pinhole while suppressing a cost increase.

An outer box 2 in which an inner gag 3 is stored is provided with a pair of inner flaps 2d and an outer flap 2e covering the outside of the flaps. A flat thin-walled part 2g which is thinner than other portions is formed at the outer peripheral edge of each inner flap 2d.