A cooling system for cooling a suppression pool water stored in a suppression pool arranged in a reactor containment vessel in which a reactor is housed, the cooling system comprising: a suppression pool water cleanup system suction line for sucking the suppression pool water from the suppression pool and for allowing the water to flow; a suppression pool water cleanup system pump installed in the middle of the suppression pool water cleanup system suction line; a fuel pool cooling and cleanup system line one end of which is connected to the suppression pool water cleanup system suction line; a filtration demineralizer installed in the middle of the fuel pool cooling and cleanup system line and configured to clean up the suppression pool water flowing in the fuel pool cooling and cleanup system line; a suppression pool water cleanup system discharge line connected to the other end of the fuel pool cooling and cleanup system line for returning the suppression pool water cleaned up by the filtration demineralizer to the suppression pool; a suppression pool water cooling line one end of which is connected to a line connecting between the suppression pool water cleanup system suction line and the fuel pool cooling and cleanup system line on an inlet side to the filtration demineralizer and the other end of which is connected to a line connecting between the fuel pool cooling and cleanup system line on an outlet side to the filtration demineralizer and the suppression pool water cleanup system discharge line; and a heat exchanger for cooling suppression pool water installed in the middle of the suppression pool water cooling line, wherein the heat exchanger for cooling suppression pool water is configured to operate when the temperature of the suppression pool water reaches a given temperature, to perform heat exchange with the suppression pool water from the suppression pool water cleanup system suction line to cool the water, and to return the cooled suppression pool water to the suppression pool through the suppression pool water cleanup system discharge line. a suppression pool water cleanup system suction line for sucking the suppression pool water from the suppression pool and for allowing the water to flow; a suppression pool water cleanup system pump installed in the middle of the suppression pool water cleanup system suction line; a fuel pool cooling and cleanup system line one end of which is connected to the suppression pool water cleanup system suction line; a filtration demineralizer installed in the middle of the fuel pool cooling and cleanup system line and configured to clean up the suppression pool water flowing in the fuel pool cooling and cleanup system line; a suppression pool water cleanup system discharge line connected to the other end of the fuel pool cooling and cleanup system line for returning the suppression pool water cleaned up by the filtration demineralizer to the suppression pool; a suppression pool water cooling line one end of which is connected to a line connecting between the suppression pool water cleanup system suction line and the fuel pool cooling and cleanup system line on an inlet side to the filtration demineralizer and the other end of which is connected to a line connecting between the fuel pool cooling and cleanup system line on an outlet side to the filtration demineralizer and the suppression pool water cleanup system discharge line; and a heat exchanger for cooling suppression pool water installed in the middle of the suppression pool water cooling line, wherein the heat exchanger for cooling suppression pool water is configured to operate when the temperature of the suppression pool water reaches a given temperature, to perform heat exchange with the suppression pool water from the suppression pool water cleanup system suction line to cool the water, and to return the cooled suppression pool water to the suppression pool through the suppression pool water cleanup system discharge line.