We report the case ofa 79-year-old Japanese woman diagnosed with human epidermal growth factor receptor 2 (HER2)-positive gastric cancer of the cardia with multiple lung metastases that showed complete response to trastuzumab-containing chemotherapy. First, we administered tegafur/gimeracil/oteracil (S-1) and cisplatin (CDDP) concurrently to the patient. Next, we switched to trastuzumab in combination with capecitabine and CDDP because gastric cancer tissue indicated HER2-positivity. Considering the patient's age and renal function, the dose of CDDP was decreased to 50% after starting the medication. Before the 2nd course of trastuzumab-containing chemotherapy, oral mucositis (Grade 3) and hand-foot syndrome (Grade 1) were observed. Therefore, a one-step dose reduction of capecitabine was necessary. After the 4th course, the primary gastric tumor was no longer visible endoscopically. After the 7th course, computed tomography (CT) showed the disappearance of all lung metastases. Accordingly, the patient was considered to be completely responsive to the medication. After the 12th course, recurrence of the tumor was not identified and at the request of the patient, the trastuzumab-containing chemotherapy was discontinued. Regular follow-up showed no evidence of recurrence 8 months after discontinuing treatment and the patient was in good condition 21 months after her initial diagnosis.