A 65-year-old male was admitted for hemorrhagic gastric ulcer. Since anti-Helicobacter pylori-immunoglobulin G antibody tested positive, eradication therapy was administered using rabeprazole, amoxicillin, and clarithromycin. During hospitalization, colonoscopy showed normal colonic mucosa except for a polyp of the sigmoid colon. He was discharged 4 days after finishing eradication therapy, but fever up and diarrhea appeared on the following day. After re-admission, colonoscopy revealed multiple yellowish-white, small circular membranous elevations, and a diagnosis of pseudomembranous colitis was made. He was successfully treated by oral administration of vancomycin. Concomitant use of antibiotics and a proton pump inhibitor for a hospitalized patient is a risk for pseudomembranous colitis. However, H. pylori eradication therapy should be started at re-introduction of oral feeding in cases of bleeding ulcers because rebleeding can be mortal in patients in 'poor general condition'. Physicians should consider pseudomembranous colitis as a diagnosis for the patients with diarrhea and high fever following H. pylori eradication therapy.