We encountered a case of synchronous bilateral breast cancer in an elderly patient. A 75-year-old woman visited our hospital because she was identified with an abnormality on mammography during breast cancer screening. An elastic hard tumor 2.0 cm in diameter was detected in the A area of her right breast. Mammography showed an irregular tumor shadow with spicula in the right breast and micro-calcifications in her left breast. Ultrasonography showed a low echoic lesion 20mm in size in the A area of her right breast and a low echoic area 5mm in size in her left breast. Histological examination of core needle biopsy specimens revealed invasive ductal carcinoma in both breasts. Synchronous bilateral breast cancer was diagnosed. Bilateral muscle-preserving mastectomy with sentinel lymph node biopsy was performed. Postoperative histology revealed bilateral invasive ductal carcinoma without lymph node metastasis that was positive for ER and PgR, was negative for HER2, and had a Ki-67-positive cell index of 20% in the right breast(T1N0M0, Stage Ⅰ), and that was positive for ER, was negative for PgR and HER2, and had a Ki-67-positive cell index of 5% in the left breast(T1N0M0, Stage Ⅰ). The surgical margins were negative. She was administered endocrine therapy as adjuvant therapy for 5 years after the surgery. Eight years after the surgery, she was well without metastasis.