Our patient was a 61-year-old man with dedifferentiated chondrosarcoma (DCS) and a benign thrombus or a tumor thrombus. We diagnosed the patient with a tumor thrombus by angioscopic observation of a smooth-surfaced, white, elevated lesion in the IVC that had invaded the vein wall. Although anticoagulant therapy was discontinued, his condition progressed without an elevation in D-dimer levels or the appearance of new thromboembolism. After one course of chemotherapy, the patient refused to continue the treatment and was treated with palliative care. He died of the underlying disease approximately half a year later. This is the first case of a DCS patient where benign and tumor thrombi was differentiated by angioscopy. Angioscopy is one of the useful tools for this differentiation which can evaluate the color tone and surface properties of an object.