Occult breast cancer, which develops as a metastatic lesion with no primary tumor detected in the breast, is a rare breast cancer. A 68-year-old female patient particularly complained of the presence of a right axillary mass. The mass in the right axilla was palpable, but no tumor was found in both the breasts on palpation, ultrasound examination, or MRI. Partial breast resection and axillary lymph node dissection were performed following a diagnosis of invasive ductal carcinoma by core needle biopsy. There was no mammary gland tissue present around the tumor due to the pathology of the disease, and the tumor was diagnosed as occult breast cancer. As the cancer was ER negative and HER2 positive, treatment with a combination of FEC, docetaxel, and trastuzumab was initiated. Radiotherapy, which irradiated the right supraclavicular fossa and the right mammary gland, was administered. No disease recurrence and mammary tumor has been reported in the patient till date. Treatment of occult breast cancer generally includes local therapy such as radiation and surgery. However, in the present case, we did not operate upon the breast; instead we treated the right breast and the right supraclavicular fossa with radiation therapy. As the tumor was HER2 positive, we reasoned that local control of disease would be likely if treatment with chemotherapy and trastuzumab was performed effectively.