Provided is a cardboard box in which perforations for separating flaps are hardly broken when the flaps are folded. In the cardboard box: a reverse ruled line (16) having a groove shape in which a front liner (1) side is recessed, and a pair of groove-like recessed portions (19a, 19b) which extend in parallel so as to have an area where the reverse ruled line (16) extends interposed therebetween and which are obtained by recessing the rear liner (2) sides are formed at each of boundary portions between upper flaps (11, 12, 13, 14) and corresponding peripheral walls (41); perforations (17) for separating the upper flaps are formed in the groove-like recessed portions (19b); and the upper flaps (11, 12, 13, 14) are folded along the perforations (17) toward the inside of the peripheral walls (41).