Lactulose is a non-digestible disaccharide formed from fructose and galactose. The objective of this study was to assess the effect of lactulose on gastrointestinal function after cesarean section. One hundred patients who underwent cesarean section at the Japanese Red Cross Katsushika Maternity Hospital were enrolled in this study. They were divided into 2 groups by randomization: (1) an L group that was treated with gelatinous lactulose (N = 48) and (2) a control group (C group) that did not receive gelatinous lactulose (N = 52). The interval between cesarean section and first postoperative flatus, defecation, and walking; appearance of symptoms of ileus; use of other medicines for stimulating bowel movement; properties and state of feces; and duration of postoperative hospital stay were compared between the two groups. The two groups did not show a significant difference in postoperative outcomes, except for the incidence of loose or watery stools (50% in the L group and 26.9% in the C group, P = 0.03). This study could not demonstrate the apparent effectiveness of lactulose in improving bowel function after cesarean section. Therefore, a routine use of gelatinous lactulose after surgery may negatively impact the patients undergoing cesarean section.