A 71-year-old Japanese woman with Sjögren syndrome, Hashimoto's disease and a 6-month history of cognitive impairment was admitted to our hospital because of consciousness disturbance and convulsion. Her convulsive seizure disappeared by intravenous administration of diazepam following carbamazepine, and conscious level became alert the next day. But, her cognitive function was persistently deteriorated, and a score of mini-mental state examination (MMSE) was 17/30 points. Magnetic resonance imaging (MRI) and single photon emission computed tomography (SPECT) of the brain did not show any abnormal findings. The electroencephalogram showed increased slow waves in bilateral parieto-occipital regions. Serum anti-thyroglobulin antibodies were elevated (1780 U/ml), but thyroid function was within the normal range. In addition, anti-NAE (NH2-terminal of α-enolase) antibodies were positive. We diagnosed Hashimoto's encephalopathy, and started steroid therapy. Her cognitive function gradually improved after steroid therapy, and convulsive seizure did not recur until 3 months later. We emphasize that Hashimoto's encephalopathy should be considered even in patients with convulsive seizure of adult onset without thyroid dysfunction.