PURPOSE: To enable an insulating sample and a sample in which a molecular ion peak can be detected only with difficulty such as a macromolecular sample to be stably measured by neutralizing electric charges accumulated in the sample by means of low-energy secondary electrons without causing electrons to be directly incident upon the sample.

CONSTITUTION: A secondary-beam-discharging plate 10 for example a plate made of a material such as Cu-Be alloy, is installed near a sample 3 and on a sample holder 4' for installing the sample 3. In analyzing secondary ions (Ip2) discharged from the surface of the insulating sample 3 by irradiating a primary ion beam (Ip1) on a spot (S) of the sample 3, a primary electron beam (Ie1) is irradiated upon the secondary-electron-discharging plate 10 in order to neutralize accumulated electric charges. As a result, secondary electrons (Ie2) are discharged from the secondary-electron-discharging plate 10 before becoming incident upon the sample surface to neutralize electric charges accumulated in the sample 3. The secondary electrons (Ie2) the energy of which is limited to 30 eV, can efficiently neutralize accumulated electric charges irrespective of the kind of the sample 3. In addition, since electrons with high speeds of not less than several hundreds eV do not become incident upon the sample surface, there is a decreased possibility that molecules of a high molecular compound or the like are severed by colliding with high- speed electrons, thereby facilitating the detection of a molecular ion peak.