An 80s man presenting with general malaise and anorexia was referred for treatment of abdominal tumor. Abdominal contrast-enhanced CT revealed a tumor in the left renal cavity. The inside of the tumor coexisted with a fat component and a solid component having a contrast effect. In addition, 2 solid tumors were found to be in contact with the stomach, away from the primary lesion. Based on these findings, retroperitoneal liposarcoma with intraperitoneal metastases was diagnosed. The patient underwent excision of the retroperitoneal tumor and local gastrectomy. The size of the main tumor was 21×18 cm, and the weight was 2.0 kg. Histopathology of the resected specimen showed dedifferentiated liposarcoma and its metastases. The resected margin of the excised tumor was negative. Liposarcoma has a high local recurrence rate, and the status of a resected margin of the tumor is an important factor for prognosis. Here, we report a case of dedifferentiated liposarcoma with metastatic lesions that could be completely resected.