To reduce labor of an operator when carrying out fraction treatment caused by a lot change, etc.

This is the fraction treating method in the case of a fractional number of boxes less than the predetermined number input to a stage stacking device in the case of packing beverage containers in the boxes by a boxing device and stacking the boxes packed with the beverage containers in stages by the predetermined number unit by the stage stacking device 10, a detected member free to detect by a sensor 16 on the side of the stage stacking device is mounted on the last box in a fractional number of the boxes, the last box 50 on which the detected member is mounted is detected by the sensor provided on the side of the stage stacking device, and stage stacking of only a fractional number of the boxes is automatically carried out by the stage stacking device after the last box is detected.