Method of forming mass image

A method of forming a two-dimensional distribution image of a target constituent in constituents of a biological sample based on mass spectra of the biological sample, comprising:

• determining a region to which the mass spectra belong, wherein the region is selected from the group consisting of an intracellular region, a nuclear region, and an extracellular region;
• calculating, for each region determined in the previous region determination, a normalized intensity of the mass derived from the target constituent from a mass signal intensity derived from the target constituent in a mass spectrum of the constituents and a mass signal intensity derived from an internal standard material in the constituents;
• and
• forming the two-dimensional distribution image of the target constituent based on the normalized intensity of the mass derived from the target constituent,
• wherein the internal standard material is selected for each of the regions.