An 80-year-old man was admitted with anorexia, a high fever and general icterus. Laboratory examinations revealed remarkable inflammation and elevation of liver/biliary enzymes. Computed tomography (CT) showed a high-density lesion in the bile duct, and endoscopic retrograde cholangiopancreatography (ERCP) disclosed an oval filling defect mimicking choledocholithiasis. Plastic stents were inserted to treat the acute cholangitis; however, the patient's symptoms recurred. An erythematous hypervascular mass obtained with a balloon catheter contained numerous pleomorphic histiocytic cells with eosinophilic cytoplasm, remarkable anisonucleosis and occasional mitoses. A diagnosis of histiocytic sarcoma (HS) was made based on the results of intensive immunohistochemistry. Monoclonal rearrangement of the IGH and TCRG genes, IGH split and IGH/BCL2 fusion was negative, although polysomy 8, 14, and 18 was detected. The patient was treated conservatively and died of the disease 20 months after the initial diagnosis. To the best of our knowledge, this is the first case of bile duct HS. This case, which involved numerical alterations of chromosomes, presented with CT and ERCP findings similar to those of choledocholithiasis.