A man in his 60 s underwent upper gastrointestinal endoscopy at our hospital, which revealed a 30-mm elevated lesion in the upper stomach (main lesion). There were several discolored, flat mucosal lesions and slightly elevated, reddish, subepithelial mass-like lesions (multiple secondary lesions) in the gastric fornix and body. Histopathological examination of several biopsied secondary lesions revealed gastric adenocarcinoma of fundic-gland type (GA-FG) or gastric adenocarcinoma of fundic gland-mucosa type (GA-FGM). The main lesion was suspected to be GA-FGM on magnifying endoscopy with narrow-band imaging. It was removed using endoscopic submucosal dissection for therapeutic and diagnostic purposes. The histopathological diagnosis of the resected lesion was GA-FGM, which was surrounded by two GA-FGM and > 30 GA-FG lesions. Total gastrectomy was considered; however, the patient declined further surgical treatment. Therefore, he was followed up with biannual endoscopy and computed tomography. At five years postoperatively, no tumor growth or metastasis has been observed.