A method for fractionating a sample solution includes the steps of setting a first mass spectrometry condition and mass range information; setting a second mass spectrometry condition and mass range information; executing a mass scan by the mass spectrum acquisition portion under the first mass spectrometry condition and obtaining first mass spectrum data; extracting first chromatogram data from the first mass spectrum data based on the first mass range information; executing a mass scan by the mass spectrum acquisition portion under the second mass spectrometry condition to obtain second mass spectrum data; extracting second chromatogram data from the second mass spectrum data based on the second mass range information; switching the first and second spectrometry conditions and repeating the mass scan cyclically; adding the first and second chromatograph data to obtain a chromatogram data; and operating the fraction collector based on the chromatogram data.