Provided is a method that achieves both of soft ionization and high ionization efficiency of a substance to be analyzed at each measurement site without impairing a two-dimensional distribution state of the substance to be analyzed in desorption ionization mass spectrometry of a substance to be measured. By applying, to a substance to be analyzed, an ionization assisting reagent including an organic acid including a functional group represented by —(CF2)COOH and having a boiling point of 150° C. or more, and a polyhydric alcohol having a melting point of 20° C. or less and a boiling point of 150° C. or more at normal pressure, the organic acid, which is a component of the ionization assisting reagent, effectively donates a proton to the substance to be analyzed, thereby improving ionization efficiency, and the polyhydric alcohol, which is a component of the ionization assisting reagent, inhibits fragmentation.