The present invention is to provide a safe antidiarrheal agent and a method for suppressing diarrhea by which an efficient antidiarrheal effect is achieved without using antibiotics or synthetic antibacterial agents that have been conventionally used against infections in livestock and poultry. Antibiotic-resistant pathogenic infections can also be prevented and treated by the present invention. The antidiarrheal agent for livestock and poultry comprising heat-killed bacteria of Enterococcus faecalis EC-12 (FERM ABP-10284) or a treated product thereof as an active ingredient is orally administered to livestock and poultry, particularly to breast-feeding sows and piglets around the weaning period.