To identify the source of contamination of raw bulk milk with Listeria, we attempted to isolate the bacteria from various samples such as cattle-related samples, bulk storage tanks and the environments on three farms. On farms A and B, Listeria monocytogenes was repeatedly isolated from raw milk, while on farm C, it was scarcely isolated from it. On the former farms, Listeria was detected in cattle-related samples and the environments. On the other hand, only one fecal sample on the latter farm was Listeria-positive. Especially, we demonstrated that the bulk tank on farm A was contaminated with L. monocytogenes. Then, L. monocytogenes was controlled by continuously washing the bulk tank on farm A with alkaline detergent.