A 49-year-old man presented with proptosis of the right eye. An x-ray film of the skull showed a lytic lesion in the right frontal bone. No other skeletal lesions were found. Immunoelectrophoretic analysis showed a monoclonal elevation of IgG level. A soft orbital tumor near the foramen of the optic nerve was removed and found to be a plasmacytoma. The patient was given local irradiation. When IgG level increased about four months after surgery, he was treated with chemotherapy. IgG level decreased and no further skeletal lesions were found.