A 53-year-old man was evaluated for a chief complaint of abdominal bloating. Physical examination revealed an abdominal distention and ascites, and CT showed multiple large retroperitoneal masses. The patient was diagnosed with retroperitoneal liposarcoma. Surgery was performed including the tumor, small bowel, and sigmoid resection, and an artificial anus was constructed. Multiple tumors in the peritoneum were noted. Large dark red tumors that were hemorrhagic were resected, but the yellowish tumors were unresectable. On histopathology, the dark red lesions showed dedifferentiated liposarcoma, and the yellowish lesions showed well-differentiated liposarcoma. One month postoperatively, peritoneal dissemination increased including nodular infiltration of the artificial anus and multiple hepatic metastases. Despite VAC chemotherapy (VCR 1.5 mg, ACD 0.5 mg, CPA 900 mg), progressive disease (PD) was noted. As second-line chemotherapy, weekly IFM (2 g)+CDDP (30 mg) was given. Shrinkage of the tumor infiltrates in the artificial anus, decreased abdominal bloating, and improved QOL were observed. A partial response (PR) against peritoneal dissemination was achieved. However, hepatic metastases increased, and the patient died 6 months after surgery. This case suggests that IFM+CDDP may be useful in dedifferentiated liposarcoma.