A 58-year-old woman visited our hospital for closer examination of a left breast tumor detected by ultrasonography in mass screening. The tumor was not palpable, mammography showed a high-density lesion with microlobulated margin, and ultrasonography showed hypoechoic oval shape tumor with an irregular border. Aspiration biopsy cytology indicated ClassII, but adenoid cystic carcinoma of the breast was diagnosed with core-needle biopsy. The patient underwent quadrantectomy of the breast and sentinel lymph node dissection. In the histopathology, the tumor had a gland-like structure and solid component. It was constituted of epithelioglandular and myoepithelial cells, and diagnosed as a adenoid cystic carcinoma of the breast. Adenoid cystic carcinoma was characterized as a structural variant, the so-called cribriform pattern, with poorly atypical small cells. Therefore, core-needle biopsy, with can diagnose a structural variant, is more useful than aspiration biopsy cytology which only diagnoses the atypical cells.