Disclosed in the present invention are a use as an immune enhancer of phytosphingosine-1-phosphate (P1P), O-cyclic P1P (cP1P), N-acetylphytosphingosine-1-phosphate (NAPS-1-P), and pharmaceutically acceptable salts thereof, and a pharmaceutical composition or vaccine for treating dementia, comprising such substance on the basis of a neuronal cell death inhibitory effect. The substance according to the present invention exhibits an effect of enhancing Th2 immune response and inhibiting Th1 immune response and has an effect of protecting neurons as well, and thus can be useful as an immune enhancer to help the production of antibodies for the development of a dementia vaccine, as well as an agent for preventing or treating dementia.