A 67-year-old woman was referred to our hospital for type 1 gastric cancer. We diagnosed the patient with advanced gastric cancer (tub2>por, HER2-positive), and the clinical findings were T4aN1M1 (lung), Stage Ⅳ. The patient had cancer cachexia. As first-line chemotherapy, a regimen consisting of capecitabine (X) (1,600 mg/m2/day, days 1-14), cisplatin (P) (60 mg/m2/day, day 1), and trastuzumab (H) (8 mg/kg/day>6 mg/kg/day, day 1) was administered every 3 weeks and repeated 6 times. After receiving 6 courses, the patient's general condition and cancer cachexia symptoms improved. The size of the primary lesion significantly decreased, and lung metastasis was not visible on a PET scan. Simultaneously, a new 0-Ⅱc tumor was detected in the cardia, and a biopsy revealed a malignant lymphoma. Total gastrectomy and D2 lymph node dissection without splenectomy were performed. The final diagnosis was T4aN0M0. After treatment, the tumor was histologically evaluated as Grade 1b, and the remnant cancer cells were found to be HER2-negative (loss of HER2). The patient has been alive for 5 months and is receiving adjuvant chemotherapy comprising capecitabine and trastuzumab. Following chemotherapy containing trastuzumab, the patient with cachexia who was diagnosed with unresectable HER2-positive advanced gastric cancer showed improvement in the cachexia symptoms and the disappearance of lung metastasis.