Idiopathic spinal cord herniation is a rare disease, few cases having been reported. We encountered a case of idiopathic spinal cord herniation presenting with severe spasticity in the right leg and urinary dysfunction. The spinal cord was herniated into a cavity created by duplication of the dura mater and resection of the inner layer improved the neurological deficits. MRI, myelography, and CT myelography were useful for diagnosing this disease. Four radiological signs of spinal cord herniation are described.