A 61-year-old woman presented with a mass in her left breast and was diagnosed with breast cancer (ductal carcinoma in sutu ; DCIS) below the nipple. There was a flat elevating skin lesion in the ipsilateral breast near the inframammary fold. She had been aware of it for 10 years at least, and we considered it as a “mole”. She underwent total mastectomy and sentinel lymph node biopsy without resecting the skin lesion. The sentinel lymph node biopsy resulted in negative and no axillary lymph node dissection was added. Histopathological diagnosis was DCIS, ER positive, PgR negative, and HER2 positive. No lesion was detected in the nipple or the skin. Three months later, she visited the department of plastic surgery with expecting the resection of the skin lesion like a “mole” and then underwent that operation. The histopathological diagnosis was Paget's disease in situ. Immunohistological findings were ER positive, PgR negative, and HER2 positive that were the same as DCIS. There was no invasive lesion in both tumors, and no macroscopic nor microscopic continuity between them. We report this very rare case of simultaneous occurrence of ectopic Paget's disease and DCIS in the unilateral breast.