A case of a 55-year-old male with herpes simplex encephalitis (HSE) was reported. He was admitted because of fever, headache and memory disturbance. T1 weighted MRI showed low signal intensity and T2 weighted imaging revealed high signal intensity in the medial portions of bilateral temporal lobes. Herpes simplex virus (HSV) antibody titer in cerebrospinal fluid (CSF) was not elevated. HSV DNA in CSF was amplified by polymerase chain reaction (PCR) and identified by the microplate hybridization method. The PCR technique would be useful for the diagnosis of HSE.