The present invention provides a method for obtaining structural information conveniently and rapidly by generating a negative ion without adding an acidic substance to matrix, thereby improving sensitivity of measurement by a mass spectrometer, and by generating structural specific ions with high reproducibility, and a method for screening disease marker and a method for analyzing a sample containing a biomolecule. A method for mass spectrometry of a sugar chain comprising the steps of: preparing a sample containing a sugar chain; labeling the sugar chain with a labeling compound, to obtain a labeled sugar chain; and subjecting the labeled sugar chain to measurement of negative ions by using a MALDI mass spectrometer, thereby conducting analysis of the sugar chain. A method for screening disease marker comprising the steps of: (1) labeling the biomolecule X derived from a subject affected by a particular disease; subjecting the labeled biomolecule X′ to measurement of negative ions by a MALDI mass spectrometer, (2) separately, labeling the biomolecule Y derived from a subject unaffected by the particular disease; subjecting the labeled biomolecule Y′ to measurement of negative ions by a MALDI mass spectrometer; and (3) comparing mass spectrum of the labeled biomolecule X′ obtained in (1) with mass spectrum of the labeled biomolecule Y′ obtained in (2) to find mass spectrum peaks which are mutually different, thereby ascertaining presence of structure involved in expression of the particular disease.