A combined cycle power plant feedwater system includes: a feed pump which supplies feedwater to a heat recovery steam generator; a first pipe which extracts part of the feedwater from a flow path in mid-course of pressurization of the feed pump; a first boiler supplied with the feedwater led into the first pipe and subjected to first water treatment; a second pipe where the feedwater discharged from a feed pump outlet flows; a second boiler supplied with the feedwater led into the second pipe and subjected to second water treatment more downstream than the flow path in mid-course of the pressurization; and a water-treating substance supply device which supplies a water-treating substance for the second water treatment to the feedwater flow path in the feed pump, at a position more downstream than a connection position of the first pipe and more upstream than a connection position of the second pipe.