A new method to measure low intensity neutrons such as secondary neutrons generated in radiotherapy has been developed. In the proposed method, thermal neutron flux is determined by measuring the conversion electrons from 134mCs generated in a CsI(Tl) scintillator irradiated by neutrons. Because the generated 134mCs is contained in the scintillator as internal sources, the emitted conversion electrons fully stop in the scintillator and counted with high efficiency. The thermal neutron flux in the neutron radiography facility of the Kindai University's research reactor, UTR-KINKI, was measured by the proposed method. The cadmium difference method was used to separate the contributions of thermal and epi-thermal neutrons. The result was in good agreement with the one obtained by the conventional gold foil activation method. It was concluded that the proposed method is useful for the measurement of thermal neutron flux.