A 62-year-old right-handed woman had presented progressive speech impediment over 4 months. She was alert without any convulsions or involuntary movements. Neurological examination showed Wernicke's aphasia, constructional apraxia. Her magnetic resonance imaging (MRI) showed an old cerebral infarction in the left parieto-occipital area, in addition to ischemic changes in the bilateral deep white matter. Electroencephalography (EEG) revealed periodic lateralized epileptiform discharges (PLEDs) predominant in the posterior left hemisphere. The PLEDs as well as the cortical symptoms improved after an administration of anti-convulsive agents, thus establishing the diagnosis of non-convulsive status epilepticus (NSE). It should be emphasized that NSE manifesting as Wernicke's aphasia should be distinguished from dementia syndrome because it is a treatable disorder.