We report the case of a 58-year-old woman who was referred to our hospital due to frequent bloody mucus diarrhea. She was diagnosed with cap polyposis based on typical endoscopic and histological findings. Colonoscopy revealed multiple, reddish, mucus-capped polypoid lesions from the rectum to the sigmoid colon. A pathological examination revealed that the polyps were covered by erosive and inflamed granulation tissue with decreased crypt cells. Laboratory data indicated positive values for Helicobacter pylori immunoglobulin G antibody and hypoproteinemia. Metronidazole, H. pylori eradication, and levofloxacin therapies were not effective; however, the subsequent administration of betamethasone enema dramatically improved the clinical symptoms and endoscopic findings. The hypoproteinemia was normalized after the therapy. The dose of the betamethasone enema was tapered gradually, and no recurrence was observed 6 months after discontinuation of the treatment. This case suggests that betamethasone enema may be considered as the second treatment choice for cap polyposis patients after H. pylori eradication, metronidazole or levofloxacin therapy.