To provide relatively high sensitivity of an analytical method while following conventional structure.

A molecular beam 3 of a measuring-objective molecule is irradiated with a flat laser beam 4 having a major axis in a molecular beam 3 flowing direction as the laser beam 4 for ionization to ionize the measuring-objective molecule, in a laser inonization mass spectrometer wherein the molecular beam 3 is irradiated with the laser beam 4 to ionize the objective molecule, so as to conduct mass spectrometric analysis by detecting an ion current therein.