Intrahepatic cholangiocarcinoma is the second leading primary hepatic tumors, accounting for 5% of all hepatic tumors. The curability depends on the operability; however, the difficulty of early diagnosis and late clinical presentation account for the poor prognosis. Therefore, development of a novel therapeutic option and a method to determine the viability of the primary tumor, which hinder the assessment of the impact of other therapies, including chemotherapy and radiotherapy are needed. Although FDG–PET has been used to detect distant metastases of ICC, which are present in 20% of patients at the initial diagnosis, little is known about the efficacy of FDG–PET of the primary lesion of ICC. Here, we present the case of a 31‐year‐old male diagnosed with unresectable ICC and successfully treated with carbon‐ion radiation, and present the usefulness of fluorodeoxyglucose–positron emission tomography in the determination of the viability of the tumor.