A counter ejector installed in the downstream section of a box-making machine and having a hopper section that receives from underneath a sheet-shaped cardboard box that has been conveyed and fed from the upstream side, and a ventilation device that blows air from the top side of the cardboard box towards the hopper section. The ventilation device is capable of changing at least the region of ventilation that is towards the conveyance direction of the cardboard box, and said ventilation region has a size in the conveyance direction corresponding to both the front and back ends of the largest box in the conveyance direction within the hopper section.