An 83-year-old woman who was admitted to other hospital, was consulted us because of continuous tarry stool and abdominal fullness. On upper endoscopy, the submucosal tumor from greater curvature of gastric fornix invaginated into the duodenal bulbus, showing so-called "ball valve syndrome (BVS)". As the tumor incarcerted again on the following endoscoopy, she underwent laparoscopic partial gastrectomy. The submucosal tumor was diagnosed histologically as gastrointestinal stromal tumor (GIST) arising from the muscularis mucosae. This is the first report that GIST arising from the muscularis mucosae in gastric fornix showed a BVS.