To provide a method with which variations in hydrogen ions or a deuterium ions and a secondary generated particles are observed through making nuclear reactor neutrinos pass along the hydrogen ion beam or the deuterium ion beam.

Anti-electron neutrinos νe* from a nuclear reactor is received by the positive hydrogen ion H+ beam which has a high-energy orientation, and the following interaction is performed: νe* + H+ (positive hydrogen ion) → e+ (positive electron) + n (neutron). In this case, the orientations of the beams of νe* and H+ are made to coincide. Acceleration is conducted so as to make the momentum of H+ become larger than that of e+. A 90-degree bend magnetic field mass spectrometer is arranged after the H+ beam. The current density of e+ is detected nearly at a total flow rate of νe* (1011/cm2 sec). A method which utilizes polarization of hydrogen molecules adsorbed spontaneously on the insulation film surface of a capacitor is used, with the H+ beam being generated easily and at low cost, and the H+ beam can be directed easily in the direction of a nuclear power plant.