Intensive radiations presumably associated with lightning activities have sometimes been detected around nuclear facilities in Japan. In order to investigate the generation of bremsstrahlung photons caused by thunderstorm electric fields, we calculated the behavior of secondary cosmic ray electrons and photons in electric fields by Monte Carlo technique. The photon flux increased greatly in the region where the field strength exceeded 280(ρ(z)/ρ(0)) kV/m, and the photon energy spectrum showed a large increase in the energy region of several MeV. These are consistent with the observed results during the winter thunderstorms.