In clinical practice, bowel sounds are often used to assess bowel motility. However, the diagnosis differs depending on the literature because diagnoses have been based on empirically established criteria. To establish diagnostic criteria, researching the mechanism of bowel-sound occurrence is necessary. In this study, based on simultaneously measured X-ray fluoroscopy and bowel sounds, correlation and Granger causality among bowel movement, luminal content movement, and abdominal sound were estimated. The results supported our hypothesis that the bowel moves luminal contents and luminal contents generate abdominal sounds.