A 70-year-old woman visited our breast clinic, because a right breast mass had been noted in enhanced computed tomography (CT) before surgery for intrahepatic bile duct cancer. A tumor was located in the D region of the right breast. It was spiculated and comparatively elastic. Mammography revealed a tumor mass with coarse calcification in the middle outer quadrant of the right breast. Initially, a fibroadenoma was suspected. Ultrasonography revealed a tumor in the CD region of the right breast that was 41×41×16 mm in size, oval shaped, and smooth-surfaced, with a heterogeneous low-echoic level inside, no interruption of the border of the mammary gland, with fine or small and coarse calcification, and a small amount of flow. Magnetic resonance imaging (MRI) also led to an initial speculation that it was fibroadenoma. Core needle biopsy resulted in a diagnosis of ductal carcinoma in situ (DCIS). Right breast partial resection was performed. Pathological findings revealed an 11 mm×8 mm DCIS in the fibroadenoma, around which usual ductal hyperplasia, columnar cell lesions, flat epithelial atypia, atypical ductal hyperplasia, papillomatosis, and sclerosing adenosis were observed.