A 71-year-old woman visited our hospital for the examination and treatment of retroperitoneal tumor. CT showed a retroperitoneal tumor extending to the posterior mediastinum; the tumor pressed the IVC and widely abutted the aorta. On MRI, the tumor showed low intensity on T1WI and high intensity on T2WI and DWI. However, the tumor did not show signal reduction on an ADC map. PET-CT showed high accumulation at the tumor. The patient was diagnosed with sarcoma arising from the retroperitoneum. The tumor located on a part of the diaphragm was resected. Histological examination revealed spindle cells with atypical nuclear and multinuclear cells. There were no lesions of well-differentiated liposarcoma. Both CDK4 and MDM2 tested positive on immunohistological staining. Histopathologically, the tumor was diagnosed as dedifferentiated liposarcoma without any well-differentiated liposarcoma component. The postoperative course was uneventful, and she was discharged on the 13th day after surgery. Two months after surgery, no recurrence has been detected.