In the removal of white blood cells from blood, the efficiency of white blood cell removal by a filter is improved. Centrifugal force is applied to blood placed in a blood bag by centrifugal separation to form a plurality of separation layers having a concentration gradient of white blood cells. The separation layers are passed through a filter in ascending order of white blood cell concentration to remove the white blood cells from the separation layers. The separation layers from which the white blood cells have been removed are received in a first blood component bag.
