The patient was a 48-year-old man in whom a slow progression in walking difficulty occurred over a year. Magnetic resonance imaging (MRI) and computed tomography myelography (CTM) revealed duplicated dura mater from T1 to T12 and spinal cord herniation in the inner layer of the dura at the T4-T5 level. Idiopathic spinal cord herniation with duplicated dura mater was diagnosed, and surgery was performed. Intraoperative findings were of an elliptical defect of about 1 cm in the inner layer of the dura at the T4-T5 level, into which the spinal cord was herniated. A 1.5-cm cephalocaudal incision was created in the inner layer of the dura, and the incarcerated spinal cord was released, resulting in resolution of gait disturbance and an excellent postoperative clinical course. We reviewed the reports of 11 cases of idiopathic spinal cord herniation with duplicated dura mater and summarized the clinical and imaging characteristics as follows: 1) A hernial orifice was found at the T4-T6 level, 2) cross-sectional MRI or CTM showed a "snowman-like" deformation of the spinal cord, and 3) symptoms were often improved by widening the hernia orifice.