Primary accessory breast cancer is extremely rare, and the diagnostic efficacy of Tc-99m-MIBI breast-specific gamma imaging (BSGI) has not been reported elsewhere. We present a case of primary carcinoma arising from dense accessory breast that was visualized with BSGI. A 43-year-old female patient with a palpable axillary mass underwent mammography, which showed dense parenchyma on both of the anatomic and accessory breasts with no abnormality. Subsequent BSGI showed no abnormal uptake in bilateral anatomic breasts, but focal abnormal uptake was noted in the accessory breast. Permanent pathologic evaluation confirmed invasive ductal carcinoma (not otherwise specified type) of the accessory breast.