A 54-year-old male visited a local physician with right dorsolumbar pain as the chief complaint. Ultrasonography revealed a tumor mass 13 cm in diameter at the lower part of the liver, and the patient was referred to our hospital. On abdominal computed tomography, uneven contrast-enhanced effects were recognized in the tumor. On magnetic rescmance imaging studies, T1-weighted images showed a hypoechoic pattern from the kidney and a weak hyperechoic pattern from the muscle. T2-weighted images showed uneven hyperecoic patterns. Uneven contrast-enhanced effects were recognized inside and on the margin of the tumor. A diagnosis of retroperitoneal tumor was made, and surgery was performed. Histopathological examination revealed a fascicular alignment of spindle cells in the area in which the myxoid matrix is seen. On the immunological special test, only vimentin was positive, which led to the diagnosis of fibrosarcoma. Fibrosarcoma originating from retroperitoneal tumor is relatively rare.