We observed three \gamma -ray bursts related to thunderclouds in winter using the prototype of anti-neutrino detector PANDA made of 360-kg plastic scintillator deployed at Ohi Power Station at the coastal area of the Japan Sea .
The maximum rate of the events which deposited the energy higher than 3 MeV was ( 5.5 \pm 0.1 ) \times 10 ^ { 2 } { / s } .

Monte Carlo simulation showed that electrons with approximately monochromatic energy falling downwards from altitudes of order 100 m roughly produced the observed total energy spectra of the bursts .
It is supposed that secondary cosmic-ray electrons , which act as seed , were accelerated in electric field of thunderclouds and multiplied by relativistic runaway electron avalanche .
We actually found that the \gamma -rays of the bursts entered into the detector from the direction close to the zenith .
The direction stayed constant during the burst within the detector resolution .

In addition , taking advantage of the delayed coincidence detection of the detector , we found neutron events in one of the bursts at the maximum rate of \sim 14 \pm 5 { / s } .