Abstract: To provide a cooling system of a reactor suppression pool capable of cooling suppression pool water and improving the safety of a reactor in the case where an event surpassing a postulated initiating event occurs, or in the case where cooling of the suppression pool water by a residual heat removable system does not function. A cooling system of a reactor suppression pool according to the present invention includes a heat exchanger for cooling suppression pool water installed in the middle of a suppression pool water cooling line, operating when the temperature of the suppression pool water reaches a given temperature, performing heat exchange with the suppression pool water from a suppression pool water cleanup system suction line to cool the water, and returning the cooled suppression pool water to the suppression pool through a suppression pool water cleanup system discharge line.