A 74-year-old man visited his local clinic complaining of abdominal pain that persisted for three days. He was diagnosed with diffuse peritonitis and was transported to our hospital. Contrast computed tomography(CT)showed gastric perforation and a tumor in the sigmoid colon with left obturator lymph node metastasis. He was diagnosed with diffuse peritonitis resulting from gastric perforation and performed emergent surgery. As the size of the gastric perforation was large, we performed distal gastrectomy and transverse colostomy. He was discharged without any complications, and a total of 6 courses of SOX with a bevacizumab regimen were administered postoperatively. CT following chemotherapy showed shrinkage of the lesion. He was admitted again for sigmoidectomy with left lateral lymph node resection and discharged from the hospital on postoperative day 8. We administered 2 courses of SOX regimen after the surgery. He remains alive with no recurrence 27months after the first surgery.