When a patient with encephalopathy has an organic brain lesion, his symptom is easily and often mistakenly attributed to that brain lesion. However, a combination of different conditions is also possible. We present a case of autoimmune limbic encephalitis combined with leptomeningeal carcinomatosis. A 57-year-old female patient was transferred to our institute with a 1-month history of seizure and aggressive behavior. Subacute onset of psychosis with multifocal T2 high signal lesions suggested autoimmune encephalitis, and high-dose steroid pulse and immunoglobulin therapy were started. However, a cerebrospinal fluid study revealed metastatic adenocarcinoma of non-small cell lung cancer, of which she was in complete remission state. Osimertinib, a third-generation epidermal growth factor receptor tyrosine kinase inhibitor, was started targeting leptomeningeal metastases while maintaining immunotherapy of rituximab and tocilizumab. Her neurological symptoms showed improvement in response to immunotherapy which lasted approximately 1 month and then deteriorated again. We concluded that her symptoms were more attributable to autoimmune encephalitis than leptomeningeal carcinomatosis, and discontinued osimertinib.