A 70-year-old man developed slowly progressive muscular atrophy in the lower limbs from age 28, followed by urinary disturbance from age 40. Neurological examination revealed bilateral severe muscular atrophy in the lower limbs with hypesthesia and hypalgesia, mild muscle wasting of both hands, urinary incontinence, and constipation. A sacral dimple was also recognized. A diagnosis of tethered cord syndrome with thick filum terminale was made by spinal magnetic resonance imaging (MRI). After the operation for untethering, urinary disturbance has been slightly improved. Although the adult-onset tethered cord syndrome with the thick filum terminale was extremely rare, the presence of congenital dermal abnormality and segmental distribution of the muscular involvement is often helpful for the clinical diagnosis of this disease. Spinal MRI is one of the most diagnostic method for this disease.