A man in his 50s was admitted to our hospital for acute visual disturbance, headache, and vomiting while driving his car. Brain computed tomography imaging revealed multiple cerebral embolisms. Transthoracic and transesophageal echocardiography on admission showed an abnormal mass echo (18 mm in length) attached at the atrial side of the anterior mitral leaflet. The other characteristics of the mitral valve did not show valvular destruction findings, prolapse, or significant regurgitation. There were no inflammatory laboratory findings and blood culture was negative. During anti-coagulation therapy, however, the abnormal mass echo diminished on follow-up echocardiography, which indicated that the mass echo was nonbacterial thrombotic endocarditis (NBTE). Further examination performed to determine the underlying cause of NBTE showed no malignancy, autoimmune disease, or abnormal coagulation status. We herein report a rare case of NBTE without apparent clinical underlying disease, and discuss the usefulness of echocardiography in the diagnosis of this disease.