Electron acceleration by static electric field within the dense atmosphere has been considered to be diffucult. From late 1990s, it became clear that lightning and thunderclouds are generating gamma-ray photons up to ~30 MeV, and considered to be originating from electron acceleration within thuderclouds. However, the detail of the electron acceleration process is not yet clear. Utilizing the gamma-ray detector technology for high-energy astrophysics, we have been observing this phenomena from 2006 in Japan. On February 2017, we detected a flash of gamma-rays associated with 2ndary neutrons and radio-active sources, which is the first clear evidence for photo-nuclear reactions taking place. On going observational approaches to this phenomena will be presented.