Provided are a cooling water system control method and a cooling water system control device, which make it possible to control the injection amount of a chemical substance into circulating cooling water and to control the supply of makeup water to the circulating cooling water in a cooling water system equipped with a cooling column and a freezing machine. The cooling water system control method is a method for controlling a cooling water system (1) which equipped with a cooling column (10) for performing heat exchange using circulating cooling water and a freezing machine (12) equipped with a condenser (24) for performing heat exchange between the circulating cooling water and a cooling medium, in which the injection amount of a chemical substance into the circulating cooling water is controlled on the basis of an LTD value calculated from the outlet temperature of the cooling medium and the outlet temperature of the circulating cooling water in the condenser (24) in accordance with the formula: [(the outlet temperature of the cooling medium) – (the outlet temperature of the circulating cooling water)], and the supply of makeup water and the discharge of the circulating cooling water in the cooling column are controlled on the basis of the electrical conductivity of the circulating cooling water.