To provide a device detecting approach or development of thunderclouds, using a space potential detecting function by electrostatic induction provided to a coherer and generating an alarm, thereby minimizing damage due to thunder.

The coherer has a function of detecting movement of a charge at a high sensitivity. This device having a very simple circuit configuration can detect the fluctuations in the space potential due to thunderclouds, by using these characteristics of the coherer. As a result, the small, lightweight thunder detecting device is provided. When such a device is used in a location in a citizen's life, many problems due to damages caused by thunder are dissolved.