PURPOSE: To detect and measure a neutral particle as an electric signal after converting it into light and to realize stable detecting operation having excellent sensitivity regardless of the degree of vacuum of an analysis part by using a scintillator as an ion detection part instead of a secondary electron multiplier tube.

CONSTITUTION: When the neutral particle is injected to the scintillator 10g of the ion detection part 2, the light emitted in the scintillator 10g propagates in an X and an Y direction to reach the end surfaces of scintillators 10b and 10f. Then, the light is converted into an electric signal by a photoelectron multiplier tube 11b arrayed in the Y direction and a photoelectron multiplier tube 11g arrayed in the X direction to output a pulse signal proportional to the quantities of light. An AND gate 14c applied with the input signal at two input terminals among AND gates 14a and 14c sends an output signal to a counter 15c and the number of pulse signals is counted up to count the quantity of the neutral particles injected into the scintillator 10g.