Apocrine carcinoma of the breast is a very rare subtype. Although it has no clinical differences from usual ductal carcinoma of the breast, it should be categorized as a subtype of breast carcinoma because the cells of apocrine carcinoma reveal characteristic abundant eosinophillic cytoplasms with intraductal apical snouting as well as round or oval nuclei and central macronucleoli. On fine needle aspiration cytology, the cells of apocrine carcinoma have a lot of similarity to benign or reactive apocrine cells of the breast. Therefore, it is difficult to make a differential diagnosis of apocrine carcinoma from mammary neoplasms with similar cytologic findings unless the subtle cytologic differences are recognized. We report the cytologic and histologic findings of a case of apocrine carcinoma in the breast of a 40-year-old female patient. After the fine needle aspiration cytology, she received the lumpectomy and lymph node dissection. The cellularity was moderate to high. The cytoplasmic borders of tumor cells of three-dimensional clusters were relatively distinctive, and the cytoplasm was abundant, eosinophilic, and granular. Although the nuclear/cytoplasmic ratio was low, the nuclei of the cells were variable in size and shape with prominent macronucleoli. Histologically, it was a typical invasive apocrine carcinoma, showing numerous cytoplasmic lysosomes and mitochondriae on electron microscopy.