Extramedullary plasmacytoma (EMP) is a very rare disease and mainly arises in the head and neck area. We herein reported a case of EMP arising in the retroperitoneal space. A 46-year-old man was referred to our outpatient clinic in November 1989 with the complaint of flank pain on the left side. Radiological examinations showed a tumor formation in the retroperitoneal space, which involved the left kidney, spleen and pancreas. Immunoelectrophoresis showed an elevation of serum IgG level and a spike of M-protein was detected in the serum protein electrophoresis. No bone lesions were detected, and bone marrow aspiration showed no abnormal cells. US-guided needle biopsy of the tumor led to the histological diagnosis as plasmacytoma of the IgG-kappa type. Following three cycles of preoperative chemotherapy (a THP-COP regimen), which resulted in a size reduction of the tumor by 40%, extensive resection of the tumor including extirpation of the left kidney, spleen, and tail of pancreas was performed. Because of tumor extension into the posterior wall of the stomach, however, the surgery resulted in incomplete resection. A total of 11 cycles of postoperative chemotherapy (THP-COP) was performed periodically for the residual tumor in the stomach. Rapid tumor spreading in addition to re-elevation of the serum IgG level, however, developed after the 11th postoperative chemotherapy, which extensively involved the stomach and intestines. The patient died of the disease 33 months after the initiation of treatment.