TTF-1 immunoreactivity in breast cancer has been reported to be rare. We experienced the patients with TTF-1 and HER2(+) adenocarcinoma in both pulmonary and breast lesions. Herein, we reported the case in which the treatment strategy of HER2(+) breast cancer was successful after the failure of the chemotherapy for advanced lung adenocarcinoma. A 60-year-old woman visited our institute with cough and shortness of breath. Inflammatory change of left breast with ipsilateral left axillary and supra-clavicular lymphadenopathies was observed by physical examinations. CT revealed a tumor in the left lung, bilateral pleural effusion and skin thickening in the breast. Histological analysis of the left supra-clavicular lymph node led to the diagnosis of lung adenocarcinoma (CK7 +, TTF-1 +, EGFR wt, no ALK and ROS1 rearrangement, PD-L1 TPS <1%, HER2 IHC 3+, ER -, PR -, mammaglobin-), which was also confirmed in the skin biopsy of left breast. The tumor was classified as clinical T4N3M1, stage IV. She received 4 cycles of CBDCA, pemetrexed and pembrolizumab. Partial response was achieved, but she experienced progressive disease due to cancerous pericarditis after 2 cycles of a maintenance therapy. The second line chemotherapy of docetaxel and ramucirumab was performed immediately. After 2 cycles, new lesions appeared in right mammary gland and left parotid gland. The histological diagnosis in a core needle biopsy of the right breast lesion was as same as in the left supra-clavicular lymph node. Because a possibility of TTF-1 positive breast cancer as a primary tumor could not be denied, we adopted the chemotherapy of trastuzumab, pertuzumab and paclitaxel according to the treatment strategy of HER2 (+) breast cancer. Partial response was obtained after 2 cycles and continued for 8 months. The tumor agnostic approach is important not only for drug development but also for daily clinical practice in the field of medical oncology.