Postherpetic neuralgia is a kind of neuropathic pain which annoys patients severely for life. Usually people recover from herpes zoster with no pain, but some patients, especially elderly people, go to the state of postherpetic neuralgia. Herpes zoster viruses destroy all the nerves from skin to dorsal root ganglion and nerves at the spinal level. After this central sensitization, abnormal ephaptic relays occur. Patients complain of allodynia, spontaneous burning pain, paroxysmal lancinating pain, and dullness on the lesion site. Now many remedies are available but a panacea does not exist. Ketamine is an antagonist of the N-methyl-D-aspartate (NMDA) receptor that has a crucial role in central sensitization. We met a 66-year-old woman who had suffered from postherpetic neuralgia for 10 years. We tried nerve blocks several times and medicated with various drugs but the outcome was not satisfactory. Lastly we tried ketamine infusion therapy and the result was excellent.