A 66-year-old female underwent esophagogastroduodenoscopy, which revealed a 17-mm gastric submucosal tumor (SMT) located in the posterior wall of the upper area of the stomach. She was referred to our hospital for further investigation because of accumulation of radioactive substance in the gastric wall by PET-CT. CT gastrography (CTG) revealed three gastric SMTs in total. Partial gastrectomy was performed; histopathologically, they were diagnosed as gastrointestinal stromal tumor (GIST), benign schwannoma, and necrotic tissue. Gastric schwannoma is very rare, and we could find no cases with both GIST and schwannoma. Additionally, the three gastric SMTs were only detected clearly via CTG. We report on the details of this case and the usefulness of CTG for the detection of gastric SMT.