 Irritable bowel syndrome IBS is a common disease and the pathogenesis of this disease includes central and peripheral mechanisms. In recent years, there were many studies suggesting that microbiota in the intestine may play an important role in the IBS.Whats more, small intestinal bacterial overgrowth SIBO may be an important pathogenic factor for IBS and the use of antibiotics may be beneficial. Therefore, the investigators intend to explore the efficacy of rifaximin for IBS-D in Chinese population. 