Low viscosity and a have low vapor pressure, without the risk of ignition, further excellent heat resistance, has a conventional hydrocarbon-based lubricating oil as compared to no way inferior any friction properties, high temperature, under vacuum, etc. to provide a lubricating oil, which can also be used for a long period of time under extremely severe conditions of. As the base oil is composed of cations and anions, a lubricating oil containing an ionic liquid ion concentration is 1 mol / dm ³ or more.