A rare case of IgG-kappa type multiple myeloma associated with multiple extramedullary tumor-forming lesions and myelomatous pleural effusion and ascites is reported. The patient suffered from sternal solitary plasmacytoma, which 5 years later developed disseminated tumor formation with myeloma cells mostly in the soft tissues. At autopsy, multiple extramedullary lesions particularly in the abdominal cavity and myelomatous pleural effusion and ascites were observed without diffuse infiltration of myeloma cells in the bone marrow. The disseminating change from solitary bone plasmacytoma to extramedullary tumor formation seen in this case seemed to be unusual but suggestive of a feature of an aggressive tendency of multiple myeloma.