A 73-year-old, male patient presented with the chief complaint of epigastric pain and received the diagnosis of extensive cholangiocarcinoma after a close examination. Extensive extension of the malignancy into the right and left hepatic ducts precluded a curative resection, and the patient received GC therapy. After 11 courses of GC over about 1 year, no new lesions or tumor progression was observed, and a bile duct mapping biopsy was performed to investigate the possibility of resection conversion. The results showed a marked decrease in atypia, and reactive atypia was diagnosed. A pancreaticoduodenectomy was performed, and histopathologically negative margins were obtained. The response to treatment was Grade Ⅱa according to the Evans classification. At 23 months after the start of treatment and 12 months after surgery, the patient is recurrence-free without adjuvant chemotherapy. Although the evidence for conversion surgery for biliary tract cancer has not been established, the long-term outcomes may be favorable.