To provide a cooling water system which can regulate cooling performance deterioration and distribute the quantity of flow to respective systems properly.

Cooling water cooled by a heat exchanger is supplied to respective coolers for cooling various apparatuses in a power generating plant via a cooling water supply pipe 13, and the cooling water which has completed cooling is returned to a cooling water pump 1 via a cooling water return pipe 14 for circulation. At this time, frictional resistance suppressing agent is injected into the cooling water in the cooling water return pipe 14 from a frictional resistance suppressing agent injecting device. As a result, pressure loss of a cooling system pipe is reduced.