We report the case of a sporadic gastric submucosal neurofibroma underneath a T1 stage cancer. A 61-year-old man underwent gastroscopy because of epigastralgia and was diagnosed as having T1 stage gastric cancer by an experienced gastroenterologist. Subsequently performed computed tomography (CT) showed poorly circumscribed wall thickening underneath the converged folds on three-dimensional images. On a dynamic enhancement study, the thickened wall was seen to be enhanced gradually from the arterial phase to the equilibrium phase. Based on these findings, we diagnosed stage T2 cancer. Total gastrectomy was performed, and the surgically removed specimen revealed that the wall thickening was caused by a submucosal neurofibroma and that cancer existed in this neurofibroma, invading the submucosa. This patient had no family history of neurofibromatosis, and so the lesion was diagnosed as early gastric cancer with a sporadic submucosal neurofibroma. Coexistence of gastric cancer and a submucosal tumor is rare, but such a case is one of the pitfalls of a CT diagnosis of T stage gastric cancer.