Differential diagnosis among several causes of axillary malignant mass is important. The most common cause of palpable malignant axillary mass is metastatic lymphadenopathy. Although carcinoma arising from ectopic breast tissue is rare, the diagnosis should be kept in mind when evaluating malignant axillary mass. In this report we present a case with carcinoma arising from ectopic breast tissue. $^{18}F$ FDG PET/CT was performed for the purpose of localizing primary breast cancer lesion and systemic evaluation. PET/CT showed hypermetabolic lesions only in the right axilla. There is no evidence of malignancy in both breasts. When nuclear physicians encounter a hypermetabolic axillary mass indicating malignant lesion without evidence of primary breast malignant lesion, carcinoma arising from ectopic breast tissue should be included in the differential diagnosis.