A reservoir tank for use in a nuclear reactor building includes a reservoir tank in which cooling water is stored. A partition part is disposed in the reservoir tank and partitions the inside of the reservoir tank into a first storage tank and a second storage tank so as to separate the cooling water in the reservoir tank. An inflow part is disposed in the partition part and allows the cooling water of the second storage tank to naturally flow into the first storage tank by the difference between the water levels of the first and second storage tanks when the water level of the first storage tank is lowered. This configuration is advantageous in that a nuclear reactor does not need to be pressurized and reheated when the nuclear reactor building is cooled due to a severe accident.