A 49-year-old Japanese man presented with a rare case of sinonasal leiomyosarcoma with left nasal bleeding for 12 months. He had no history of irradiation or malignancies, including retinoblastoma. Preoperative histological examination suggested vascular leiomyoma. Complete resection with endoscopic surgery was performed. Histological examination during the operation suggested that the tumor was a leiomyoma. However, immunohistochemical staining for α smooth muscle actin and desmin were helpful in establishing a definitive diagnosis of sinonasal leiomyosarcoma. The resection margins were positive for tumor cells. Staging with CT of the neck and thorax, ultrasound of the abdomen, and MRI of the head ruled out metastases. Second endoscopic tumor resection surgery was performed for positive resection margins. The patient's condition was successfully managed with additional excision, and he remains well with no evidence of recurrence and metastasis 36 months after treatment. Endoscopic management should be considered in suitable cases.