Disassembling device for cardboard box and disassembling method for cardboard box

A disassembling device 1 for a cardboard box according to the present invention is a disassembling device for a cardboard box A0 that disassembles an assembled cardboard box A0 into a flat developed state, the disassembling device 1 being provided with: two conveyors 3 and 5 that are arranged so that a carrying-in side where a cardboard box A0 is carried in becomes wide and a carrying-out side where a cardboard box A1 is carried out becomes narrow; and a pair of compressing and carrying-out rollers 7 and 9 that is provided downstream of the carrying-out ends of the two conveyors 3 and 5. Conveyance speeds V of the two conveyors 3 and 5 are set at different speeds V1 and V2.