Case 1 was a 70-year-old man, and case 2 was a 65-year-old woman. Both were diagnosed with type 2 advanced HER2-positive gastric cancer. Capecitabine, cisplatin, and trastuzumab (HXP therapy) were administered to both patients. However, both patients developed peritonitis caused by gastric cancer perforation during HXP therapy on day 38 for case 1 and day 8 for case 2. Emergency omentum filling and gastric segmental resection were performed for both patients. The same chemotherapy regimen was continued after the surgery, and partial response was observed in both patients. Because most advanced HER2-positive gastric cancers are ulcers, we should always consider the risk of gastric cancer perforation while administering HXP therapy, which has a high cytoreductive effect. Good convalescence can be expected by continuing chemotherapy after emergency surgery due to gastric cancer perforation.