Most patients with plasmacytoma demonstrate a generalized disease at diagnosis such as multiple myeloma (MM). However, 3-5 percents of patients with plasmacytoma present with either a single bone lesion, or less commonly, a soft tissue mass, of monoclonal plasma cells: solitary plasmacytoma of bone (SPB) or extramedullary plasmacytoma (EMP). Moreover, SPB has a high risk of progression to MM. We report a 65-year-old female with SPB that only involved the skull base bone. She complained of discomfort of the throat for one month and endoscopic examinations showed a submucosal tumor of the nasopharynx. The tumor was biopsied by an endoscopic transnasal approach with the navigation system under general anesthesia. Resected tumor tissue showed monoclonal proliferation of CD138-positive plasma cells with lambda light chain expression in the cytoplasm. Although a small M protein was present in serum, CT, MRI and FDG-PET demonstrated that the osteolytic lesion arising from the clivus of the skull base bone gave rise to a submucosal tumor of the nasopharynx without skeletal lesions and other related organ or tissue impairment. Bone marrow did not contain monoclonal plasma cells, which is not consistent with MM. Finally, we diagnosed the patient as having SPB, but not EMP, without complication of MM.