The present invention provides a method for mass spectrometry of phosphorylated peptides or sugar chains, which suppresses the desorption of an unstable site during ionization and achieves high sensitivity (i.e., detects molecular-related ions at a high ionic strength and at a relatively higher ionic strength than other ion species derived from the desorption of an unstable site). A method for mass spectrometry of phosphorylated peptides or sugar chains, the method comprising using, as a liquid matrix, an ionic liquid comprising a 3-aminoquinoline ion and a p-coumaric acid ion. The liquid matrix comprises 3-aminoquinoline and p-coumaric acid, for example, in a molar ratio of 5:1 to 20:1. Ammonium phosphate may be used as an additive.