We experienced a case of mucin-producing cholangiocarcinoma with inflammatory wall thickening for 5 months. A 46-year-old man who had a stomach punch ulcer, was found to have a cyst by chance on CT. After 5 months, the diameter of the cyst on CT had decreased, the wall thickness increased and the border was unclear. We diagnosed mucin-producing cholangiocarcinoma and found it invading the surrounding liver. So we conducted left hepatectomy and caudate lobectomy. Pathologically, mucin-producing cholangiocarcinoma was confirmed, but the wall thickening was due to inflammation not cancer invasion.