The patient was a 14-year-old girl who had become conscious of the difference between a part of the left nipple and breast cross section since 1.5 years before. She visited a nearby hospital and was suspected to have a tumor mass in the left breast. Thereafter, she was admitted to our hospital. On palpation, we felt an elastic and hard 5 cm mass in her left breast. We observed an apparently circumscribed mass with high density on the mammographic image and an 8-cm mass with a clear margin in the major axis on breast ultrasonography. The image of the tumor mass showed a mixed pattern accompanying a nipple-shaped solid part that protruded into the lumen. We suspected an intracystic tumor or papilloma on preoperative diagnosis. However, because we observed an outgrowth of fibrillary tissue into the cyst wall and both mammary gland tissue and fibrillary interstice in the solid part on histopathologic examination, the patient was given a final diagnosis of juvenile breast fibroadenoma. Upon satisfactory progress, the patient was discharged on the 5th hospital day, and no recurrence has been observed to date, i.e., 7 years after surgery.