A 61-year-old woman presented at a nearby clinic with a complaint of a mass in the right axilla. Initial imaging examinations, including mammography, ultrasonography, and breast MRI, did not reveal any obvious intramammary lesions, although a swollen lymph node was observed in the right axilla. Fine-needle aspiration cytology confirmed malignancy. Hence, a core needle biopsy was performed. The results indicated a suspected metastasis of invasive ductal carcinoma(ER-, PgR-, HER2-); however, the primary tumor could not be definitively determined. Despite an extensive whole-body examination, the primary tumor remained unidentified. Nonetheless, metastasis of occult breast cancer in the right axillary lymph node was postulated. Subsequent axillary dissection revealed metastases in only one lymph node. Taking the clinical findings into consideration, the patient was diagnosed with right occult breast cancer, and chemotherapy and radiotherapy were planned.