A 58-year-old man underwent cholecystectomy and partial resection of the stomach with the preoperative diagnosis of acute cholecystitis and submucosal tumor of the stomach. The submucosal tumor was found postoperatively to be a T3 stage gastric cancer with well-differentiated phenotype through histopathologic examination of the resected specimen. The patient rejected a subsequent offer of either reoperation or chemotherapy, and underwent close follow-up. Serum tumor markers rose a few months later, and cancer recurrence was confirmed by the finding of a measurable peritoneal metastasis by computed tomography. He was treated with single agent S-1, obtained a complete response 10 months later, and went on to receive the drug for 42 months. He remains disease-free for over 30 months after cessation of S-1. S-1 is recommended as a first-line chemotherapy for recurrent gastric cancer, but the treatment schedule and follow-up schedule after obtaining a complete response remain an issue.