Serratia marcescens has been reported to cause infective endocarditis among intravenous drug users, but it is extremely rare in non-intravenous drug users in Japan. In this article, we report an 85-year-old woman with diabetes mellitus who presented with low-grade fever and general fatigue. She was administered intravenous prednisolone under a diagnosis of right Bell's palsy before this admission. Blood cultures revealed positive Serratia marcescens, which was complicated by multiple cerebral infarctions after admission. Transthoracic echocardiography on day 5 revealed vegetation on the mitral valve, which was diagnosed as infective endocarditis. An operation could not be performed because of the presence of multiple cerebral infarctions. She died on day 65 because of uncontrolled heart failure.