  A report is made on a comprehensive observation of a burst-like $\gamma$-ray
emission from thunderclouds on the Sea of Japan, during strong thunderstorms on
2007 January 6. The detected emission, lasting for $\sim$40 seconds, preceded
cloud-to-ground lightning discharges. The burst spectrum, extending to 10 MeV,
can be interpreted as consisting of bremsstrahlung photons originating from
relativistic electrons. This ground-based observation provides first clear
evidence that strong electric fields in thunderclouds can continuously
accelerate electrons beyond 10 MeV prior to lightning discharges.
