A method of performing laser desorption/ionization mass spectrometry based on ions generated by exposing a sample supported on a substrate to laser light, the sample being to be subjected to a spectrum analysis, the method comprising the steps of:
- (a) causing a part of the ions to be generated through one of an interaction between the laser light and a surface of the substrate and an interaction between the laser light and an interface between the substrate and the sample;
- and
- (b) determining the generated part of the ions to be index ions and identifying a signal to become noise in the laser desorption/ionization mass spectrometry using a signal of the index ions, thereby performing the spectrum analysis without an effect of the noise.