The case herein pertains to a 78-year-old man, who was referred to our department and in whom esophagogastric junctional cancer, triggering weight loss, was indicated. A 0.75 xx circumferential Type 3 lesion was observed in the esophagogastric junction via upper gastrointestinal endoscopy, and a biopsy revealed a moderately differentiated adenocarcinoma with overexpression of human epidermal growth factor type 2 (HER2). Contrast-enhanced computed tomography (CT) showed multiple enlarged lymph nodes on the lesser curvature of the stomach. The preoperative diagnosis was T4a (SE) N2M0, Stage IIIB esophagogastric adenocarcinoma. Four courses of preoperative chemotherapy consisting of capecitabine plus cisplatin and trastuzumab were administered. The primary lesion showed cicatrization after chemotherapy, and enlarged lymph nodes were not visible on a CT scan. Total gastrectomy, splenectomy, and D2 lymph node dissection were performed. Only a small amount (approximately 5 mm) of the primary lesion remained beneath the abdominal and esophageal mucosa, and the treatment effectiveness was Grade 2. The final diagnosis was T1b (SM) N0M0, Stage IA esophagogastric adenocarcinoma. Survival with no recurrence was confirmed 6 months after surgery. These findings suggest that inclusion of trastuzumab in preoperative chemotherapy regimens improves the degree of curability of HER2-positive esophagogastric junction cancers.