The prognosis of solitary plasmacytoma varies greatly, with some patients recovering after surgical removal or local fractional radiation therapy, and others progressing to multiple myeloma years later. Primary detection of progression to multiple myeloma is important in the treatment of solitary plasmacytoma. There have been several analyses of the risk factors involved in the early progression to multiple myeloma. We describe one case of solitary plasmacytoma of the lumbar vertebra that was treated with surgical decompression with stabilization and additional radiotherapy. The patient had no factors associated with rapid progression to multiple myeloma such as age, size, immunologic results, pathological findings, and serum free light chain ratio at the time of diagnosis. However, his condition progressed to multiple myeloma less than two months after the initial diagnosis of solitary plasmacytoma. We suggest that surgeons should be vigilant in watching for rapid progression to multiple myeloma even in case that the patient with solitary plasmacytoma has no risk factors for rapid progression to multiple myeloma.