A box packing device provided with a dropping part for dropping a box, which has an annularly formed side face portion having four side faces and four tabular lid portions each extending from one of the side faces, in a state in which one of the four side faces is directed downward and an upper lid portion extending from the upper side face among the four lid portions and first and second lateral lid portions extending from the laterally positioned side faces among the four lid portions are opened outward, so that the height position of the box is changed: characterized in that the dropping part has an upper guide mechanism and a lower guide mechanism arranged below the upper guide mechanism, the upper guide mechanism includes a first guide portion (22, 23) arranged so as to oppose front faces of the first and second lateral lid portions when the box drops, and a second guide portion (24, 25) arranged so as to oppose rear faces of the first and second lateral lid portions when the box drops, and the lower guide mechanism includes a third guide portion (31, 32) arranged so as to oppose the front faces of the first and second lateral lid portions when the box drops and after the box drops, a fourth guide portion (33, 34) arranged so as to oppose the rear faces of the first and second lateral lid portions when the box drops and after the box drops, and a first drive unit for driving at least either of the third guide portion or the fourth guide portion (33, 34) toward the other after the box drops. characterized in that the dropping part has an upper guide mechanism and a lower guide mechanism arranged below the upper guide mechanism, the upper guide mechanism includes a first guide portion (22, 23) arranged so as to oppose front faces of the first and second lateral lid portions when the box drops, and a second guide portion (24, 25) arranged so as to oppose rear faces of the first and second lateral lid portions when the box drops, and the lower guide mechanism includes a third guide portion (31, 32) arranged so as to oppose the front faces of the first and second lateral lid portions when the box drops and after the box drops, a fourth guide portion (33, 34) arranged so as to oppose the rear faces of the first and second lateral lid portions when the box drops and after the box drops, and a first drive unit for driving at least either of the third guide portion or the fourth guide portion (33, 34) toward the other after the box drops. the dropping part has an upper guide mechanism and a lower guide mechanism arranged below the upper guide mechanism, the upper guide mechanism includes a first guide portion (22, 23) arranged so as to oppose front faces of the first and second lateral lid portions when the box drops, and a second guide portion (24, 25) arranged so as to oppose rear faces of the first and second lateral lid portions when the box drops, and the lower guide mechanism includes a third guide portion (31, 32) arranged so as to oppose the front faces of the first and second lateral lid portions when the box drops and after the box drops, a fourth guide portion (33, 34) arranged so as to oppose the rear faces of the first and second lateral lid portions when the box drops and after the box drops, and a first drive unit for driving at least either of the third guide portion or the fourth guide portion (33, 34) toward the other after the box drops.