We experienced a rare case of transdural herniation of cauda equina caused by increased pressure with spinal subdural extra-arachnoid hygroma (SSEH) following lumbar microsurgical decompression. A 68-year-old woman presented with complaints of right leg pain and intermittent claudication. By the diagnosis of L2/3 lumbar spinal stenosis, microsurgical decompression was performed. The surgery was successful with no issues arising such as damage to the dura mater. Lumbar magnetic resonance imaging (MRI) performed 8 days after the surgery confirmed asymptomatic SSEH on the ventral side of the cauda equina. However, posterior cervical pain and lower back pain developed 32 days after the surgery. Lumbar MRI demonstrated that SSEH had markedly increased and advanced from the lumbar spine to the cranium, compressing the spinal cord posteriorly. In addition, herniation of the cauda equina was confirmed in the dura. An emergency surgery was performed. The herniated cauda equina was fully positioned in the dural sac, and the arachnoid membrane with accumulation of spinal fluid on the ventral side was fenestrated. Immediately after the surgery, the patient's symptoms disappeared. Sufficient caution is required regarding the possibility of SSEH associated with spinal failed back surgery syndrome as it can become excessively enlarged, leading to a poor prognosis.