The patient was a 78-year-old man. Gastric cancer, type 3, was diagnosed by endoscopy in the subtotal stomach used for posterior mediastinal reconstruction after resection of thoracic esophageal cancer. Surgery was not considered to be feasible in this case because of cerebral infarction and decreased pulmonary functions; instead, the patient received TS-1 chemotherapy. Drug administration was started at the dose of 100 mg/day, one level lower than the standard dose of TS-1. Reduction in tumor size was noted endoscopically during the first course of treatment. At the end of the 4th course of treatment, the ulcerous lesion was found to have disappeared almost completely, and only mild mural irregularity was noted. The incidence of gastric cancer in the stomach tissue used for mediastinal reconstruction after esophagectomy has been reported to be 0.8%. In many of these cases, the cancer is already advanced at the time of diagnosis, precluding surgical resection. In this situation, chemotherapy with TS-1 is expected to be an effective method of treatment that can be administered at home in elderly patients with a variety of complications.