A very rare case of muco-epidermoid carcinoma of the breast is reported. A 54-year-old woman noticed a breast lump and consulted the Cancer Institute Hospital, Tokyo. Physical examination revealed a mass with an unclear margin in the upper outer area of the left breast. Both mammography and ultrasonography showed an irregularly shaped mass, suggesting invasive carcinoma. Aspiration cytology revealed solid clusters of two types of cancer cells: ones with mucus in the cytoplasm, and others without mucus. Cytological features resembled muco-epidermoid carcinoma of the salivary gland but it was not possible to establish a diagnosis of the histological type because muco-epidermoid carcinoma of the breast had not previously been encountered by the authors. After incisional biopsy, left mastectomy with axillary lymph node dissection was performed. Histologically the tumor was surrounded by a fibrous pseudocapsule and was composed of intraductal carcinoma foci and invasive cancer nests. Intraductal carcinoma foci were the low papillary type. Stromal invasion showed many solid nests composed of mucus-containing and squamoid cancer cells. There was no common invasive ductal carcinoma component in this tumor. These histological features confirmed the diagnosis as muco-epidermoid carcinoma of the breast. This is the first report of muco-epidermoid carcinoma of the breast in Japan.