A packaging box (1) includes a storage part (2), an inner flap (21) and an outer flap (22). The inner flap (21) is provided with a slit (31) along an edge (11a) and the outer flap (12) is provided with an insertion piece (41) connected to a tip edge (22a) via a bending line (42). The insertion piece (41) is inserted into the slit (31) by being bent along the bending line (42). The insertion piece (41) has an engaging piece (45) configured to be foldable along a folding line (44) extending in a direction crossing the tip edge (22a) and to have an engaging edge (45a) facing the tip edge (22a). When the engaging piece (45) is folded along the folding line (44), the engaging edge (45a) is inclined downward toward its distal end.