We report a 27 years-old previously healthy male admitted to a psychiatric hospital because of abnormal behavior. He was suspected meningoencephalitis with fever, abnormal sweating, muscle tone, confusion, and introduced to the neurology department of our hospital. After admission, increasing convulsions and apnea attack required mechanical ventilation therapy. Anti-N-methyl-D-aspartate( NMDA) - receptor encephalitis was diagnosed based on positive (20-fold) anti-NMDA antibody in cerebrospinal fluid examination. An enhanced chest computed tomography (CT) showed a 43 mm cystic mass with calcification of the anterior mediastinum. He underwent the tumor resection under median sternotomy on the 18th hospital day. The plasmapheresis and steroid therapies were treated after the operation. The consciousness level gradually improved, the patient was withdrawn from the respirator on the post operative day( POD) 35, and transferred to a rehabilitation hospital on POD 60. The pathological result was mature teratoma. However, no specific findings such as inflammatory cell infiltration into nerve components were observed. Anti-NMDA receptor encephalitis was established by Dalmau in 2007 as encephalitis associated with ovarian teratoma. It presents mainly in young adult women with psychiatric symptoms, and requires mechanical ventilation management due to disturbance of consciousness, convulsions, and central hypoventilation in a short period of time. It presents severe symptoms in the acute phase and shows a unique clinical finding with a good prognosis even though it shows a protracted course. Treatment requires prompt tumor detection and early resection, as well as methylprednisolone (mPSL) pulse, plasmapheresis, and high-dose gamma globulin therapy. It is a neurological disease that requires emergency response, and the understanding and prompt response of related departments is important.