Abstract: A cooling system for cooling nuclear fuel stored in a pool that can prevent a lowering of water level resulting from evaporation is provided. The cooling system cools the nuclear fuels 3 stored in a bottom of the pool 2 filled with cooling water 4. The cooling water 4 comprises an upper layer formed in a water surface as side in which a density of the cooling water 4 is low, a lower layer formed in the bottom side of the pool 2 in which a density of the cooling water 4 is high, and an intermediate layer in which a density of the cooling water 4 is in between those of the cooling waters 4 in the upper layer and the lower layer. A lower end 5a of a heat pipe 5 adapted to transport heat in the form of latent heat of working fluid is situated at a site where the heat is exchanged with the cooling water 4, and an upper 5b end of the heat pipe 5 is exposed to external air. The nuclear fuel 3 is stored within the lower layer.