A case sealing device is disclosed. The case sealing device (33) conveys a cardboard box (B) and simultaneously closes a flap group provided on edges of opening of the cardboard box. The case sealing device includes a folding member (370), a drive unit (375), and a control unit (40). The folding member (370) contacts a flap (Zfal, Zfar) of the top flap group (Zfa) that extends along a conveyance direction of the cardboard box (B) and folds the flap (Zfal, Zfar). The drive unit (375) moves the folding member (370). The control unit controls (40) the drive unit(375). In operation of folding the flap (Zfal, Zfar), a longitudinal direction of the folding member (370) is inclined with respect to the conveyance direction such that the folding member (370) contacts a front edge of the flap (Zfal, Zfar) in the conveyance direction before other portions of the flap (Zfal, Zfar).