Hemimegalencephaly is a rare congenital disease that occurs with intractable epilepsy and is a childhood developmental disorder. A functional hemispherectomy is indicated for the treatment of hemimegalencephaly with intractable epilepsy. We present a case of hemimegalencephaly in a 6-month-old male. After hemispherectomy, his seizures disappeared completely and postoperative neurological examination showed right hemiplegia. His right arm and limb function were recovered gradually by rehabilitation with passive movement. We investigated cortical activation using near-infrared spectroscopy (NIRS). Serial NIRS showed right cortical activation by passive movement of his right arm. We suggest that NIRS showed the ipsilateral reorganization process as an effect of neurorehabilitation for disconnection of the brain.