To exchange heat twice a day by flux and reflux of tides without using an electric power.

A nuclear power plant includes multiple safety devices, for example, storage batteries and power generators as auxiliary power supplies, a water tank, a turbine pump, a reservoir, an auxiliary circuit and the like. However, it is not safe enough in a current situation having been destroyed by an earthquake and tsunami. The present invention is provided as one of means for solution. The nuclear power plant is installed below sea level and use of the circulation device can definitely improve the safety in emergencies.