A case of solitary fibrous tumor (SFT) in the pelvic cavity with hypoglycemia is reported. The patient was a 60-year-old man who was referred to our hospital for a closer examination of hypoglycemia. Computed tomography demonstrated a mass, measuring 14 x 9 cm in size, in the pelvic cavity. Magnetic resonance imaging showed the mass to have a low signal intensity on T1-weighted images and a high intensity on T2-weighted images. Laparotomy revealed no peritoneal dissemination nor lymph node metastasis. An en bloc excision of the tumor was performed with a good recovery, and the hypoglycemia disappeared. Histologically, the tumor was composed of spindle-shaped and oval cells in sarcoma, based on a moderate mitotic rate and cellularity. Immunohistochemically, the tumor was positive for CD34 and negative for keratin, alpha-smooth muscle actin, desmin, S100 protein, c-kit protein, and epithelial membrane antigen. Based on these findings, the tumor was diagnosed to be malignant SFT in the pelvic cavity.