Candida can be a part of normal human flora, but can be harmful in immunocompromised patients. While oral or esophageal moniliasis can be found in healthy people or gravely ill patients, ulceration of the duodenum is far less common. Acid-reducing drugs such as H2 receptor blocker increase growth of microbes, including fungus, and candidal invasion is regarded as rare one of the factors that are associated with failure to respond to H2 receptor blocker therapy. Recently, we experienced a case of atypical duodenal ulcer which was found in a patient with iatrogenic Cushing's syndrome and diabetes mellitus and failed to respond to H2 receptor blocker therapy. Fungal spores and hyphae were histologically identified from the biopsy specimens that were obtained from the margin of the duodenal ulcer, and this fungus was confirmed as Candida albicans by blood culture. It should be considered that in immunocompromised patients, gastroduodenal ulcers can be caused by fungal infection.