Increases of environmental gamma-ray dose which seem to originate from lightning activity were observed around a nuclear facility in Japan. Dose increases measured by thermoluminescent dosimeters (TLDs) exposed during a period including a lightning flash were up to about 0.1 mGy. Dose rates indicated by environmental radiation monitors around the facility also increased transiently at that time. From pulse-height analysis of an NaI(Tl) scintillation detector, the photon energy spectrum evaluated by spectral unfolding is continuous with energy up to several MeV, consistent with bremsstrahlung emission from energetic electrons. Furthermore, energy spectra of bremsstrahlung photons emitted from high-energy electrons are obtained by Monte Carlo calculation. These results are discussed in conjunction with observation.