Adenomyomatous hyperplasia is most commonly found in the stomach, gallbladder, duodenum, and jejunum, while it is rarely found in the extrahepatic bile duct. A 62-year-old woman was referred to our institution with a diagnosis of common bile duct (CBD) stenosis which had been detected by endoscopic retrograde cholangiopancreatography (ERCP). Abdominal computed tomography with contrast medium revealed a thickening of the wall of the lower CBD, and this lesion was weakly enhanced by contrast medium in the arterial phase. ERCP revealed a 15-mm-long stenosis of the lower CBD, but no malignant cells were detected by either bile cytology or brush cytology. Because CBD cancer could not be ruled out, pylorus-preserving pancreatoduodenectomy was performed. Histopathologically, multiple hyperplastic glands without cellular atypia were present in the lower CBD wall. An immunohistochemical study showed fibroblasts with positive staining for alpha-smooth muscle actin surrounding the glands. The lesion was diagnosed to be adenomyomatous hyperplasia of the CBD. When a diagnosis of adenomyomatous hyperplasia of the CBD is difficult to make both preoperatively and intraoperatively, then a radical surgical procedure, such as a pancreatoduodenectomy, may be an effective treatment alternative.