We report a case of Legionella pneumophila pneumonia in a patient with interstitial lung disease. Intensive environmental investigations revealed that a system of all-day-running bathwater was the source of infection. In this case, the concentration of L. pneumophila in the hospital bathwater was low. We therefore emphasize that even a low concentration of L. pneumophila in environmental water can cause serious infections to immunocompromised patients in a hospital.