A new interface is interposed between a liquid chromatograph and a mass spectrometer, and it consists of a nebulizer, an evaporator and a cooler. An effluent from the liquid chromatograph is nebulized by the nebulizer. The nebulized effluent is heated by the evaporator, and has only its solvent evaporated. A mixed fluid which consists of the evaporated solvent and the nebulized sample is guided to the cooler, and is cooled therein. At this time, the evaporated solvent is condensed again and is separated from the nebulized sample as a liquid. The nebulized sample left behind is guided to an ion source of the mass spectrometer.