It is well known that Thorotrast, a kind of oily contrast medium, remains in the liver for a long time. Here we report a case of foreign body in the oral floor, which was at last revealed to be on oily contrast medium. The patient, a 44-year-old woman, came to our department because of uncomfortable feeling in the oral floor. As a past history, she had received an operation on sialoadenoectomy 10 years ago. Roentgenographically, some radioopaque substances were observed in the oral floor. It was revealed that these substances consisted of protein and triolein. Therefore, we diagnosed this foregin body as an oily contrast medium which had been used in the previous clinic.