Biliary cystadenocarcinoma is a rare tumor which has a better prognosis after complete surgical removal. However, since there is no other effective therapeutic option instead of surgery, the prognosis is very poor if surgical resection is impossible. We report the case of a 78 year-old woman who was treated only by hepatic arterial infusion (HAI) chemotherapy of cisplatin (85 mg) because liver functional reserve was too bad to perform a major hepatectomy. Huge biliary cystadenocarcinoma occupying the right lobe of the liver measuring 12 cm in size before chemotherapy was drastically decreased to 2 cm in size 8 months after chemotherapy. Although she had abdominal pain and anorexia before chemotherapy, she is doing well and has no complaints 9 months after chemotherapy. In conclusion, an effective therapy may be possible using hepatic arterial infusion chemotherapy of cisplatin in patients with biliary cystadenocarcinoma for whom surgical treatment is not suitable.