A 52-year-old man noticed physical weariness and anorexia in November 2008. Severe anemia was found. Pneumomediastinum was detected using x-ray and CT scanning. Esophagogastroduodenoscopy revealed a submucosal tumor with a deep ulcer in the anterior wall of the upper stomach, causing anemia. Partial gastrectomy was performed because microperforation was suspected. The resected specimen showed spindle-shaped tumor cells that were c-kit positive and CD34 positive on immunohistological staining, suggesting a diagnosis of gastrointestinal stromal tumor (GIST). Although emphysema in the abdominal cavity was not observed, air leakage from gastric GIST with necrosis in the upper stomach was inferred as the cause of pneumomediastinum.