A drinking water additive for birds which can inhibit birds for meat from reducing their weights due to stoppage of feeding prior to slaughter and can diminish intestinal harmful bacteria in birds for meat and laying hens, represented by bacteria belonging to the genus Salmonella , during stoppage of feeding, etc., and which comprises a living microorganism belonging to the genus Bacillus and a monosaccharide. The administration method comprises orally administering this additive to birds. This method is used for inhibiting birds for meat from reducing their weights or for diminishing intestinal harmful bacteria in birds for meat or in laying hens.