We recently experienced a case of idiopathic spinal cord herniation in the upper thoracic spine with development of severe unilateral leg pain: a 57-year-old man who had suffered from severe leg pain for about 15 years with lack of abnormality in his lumbar spine. On MRI of the thoracic spine, however, the spinal cord at T2-T3 was bent forward with dilatation of the posterior subarachnoid space. In combination with the findings of computed tomographic myelography, we diagnosed idiopathic spinal cord herniation and performed a surgical treatment. Immediately after the operation, his leg pain disappeared. Therefore, we concluded that the cause of his leg pain was herniation of the thoracic spinal cord.