To enable analysis of a sample of a low concentration by improving detection sensitivity.

The amount of a radiation source 16 that is a radioactive isotope element enclosed in a detection cell 11 is set in the range of not less than 20MBq and less than 100MBq that is lower than before, for instance, at 90MBq. Thus, the amount of free electrons generated following positive ionization of inert gas in the detection cell 11 is reduced, so that the change in the amount of electrons captured when an object substance (electrophilic molecule) to be detected of low concentration is introduced is more easily indicated in a detection signal and thus the detection sensitivity is improved.