Solitary fibrous retroperitoneal tumor is rare. We present a case with infiltrative growth in a 56-year-old female patient whose initial symptom was palpable tumor in the lower abdomen. Computed tomography and magnetic resonance imaging indicated a mass in the retroperitoneum under the left kidney with a poorly demarcated infiltrative growth. Surgical findings revealed a gelatinous tumor in the retroperitoneum, which had invaded up to the fatty tissue surrounding the Gerota's fascia and to the fatty tissue surrounding the descending colon. However, as there was no invasion into the Gerota's fascia, it was possible to preserve the left kidney. Pathohistological examination revealed increased cellularity in the tumor tissues as well as tissues with atypical nuclei of the tumor cells with some cell division. Due to these findings, it was diagnosed as malignant solitary fibrous tumor. Only surgical treatment was performed and the patient is alive without recurrence 2 years and 4 months after surgery.