A 73-year-old male was transferred to our hospital because of elevated serum hepatobiliary enzymes. Radiological studies showed polypoid tumor in the distal bile duct, for which malignancy could not be denied. Pancreatoduodenectomy specimen showed a 30mm-sized, friable and polypoid tumor at the distal bile duct. Histologically, tumor comprised of spindle to round mononuclear tumor cells with abundant osteoclast-like giant cells. Immunohistochemical study showed positivity for SMA and Vimentin/negativity for desmin, CD34, Histone3.3G34W and ALK. The labeling index of Ki-67 was 5-30%. At the tumor base, biliary intraepithelial neoplasia (BilIN) was seen without invasive adenocarcinoma component. Molecular study showed the tumor lacked MYHii USP6 fusion, which is often seen in nodular fasciitis. Polypoid tumor was KRAS: mutated, while BilIN component was KRAS: wild. Pathological diagnosis of anaplastic carcinoma with osteoclast-like giant cells (AC with OC) was finally made. Due to the lack of nuclear atypia and invasive adenocarcinoma component, authors felt the pathological diagnosis for the present case was very difficult. AC with OC of the bile duct is very rare, and, so far, its prognosis and details are unknown. Studies with more cases are necessary.