To analyze the structure of a sialooligosaccharide from mass spectral information by ionizing a sialooligosaccharide derivative, obtained by causing to 2-aminopyridine to act on the reductive end of sialooligosaccharide, through impact ionization thereby detecting sialooligosaccharide and determining the molecular weight.

2-aminopyridine is caused to act on the reductive end of sialooligosaccharide, as pretreatment, to produce a sialooligosaccharide derivative which is then ionized by impact ionization. Multiple product ions produced through collision of parent ions generated through ionization against an inert gas, e.g. argon or helium, are separated depending on the mass number/charge number (m/z) and then the intensity of each ion is recorded and arranged according to the order of m/z. Structure of sialooligosaccharide, e.g. arrangement of compositional unit saccharide, branch position, can be analyzed in detail based on useful mass spectral information, e.g. mass difference between respective fragment ions on the reductive or nonreductive end side.