In the technical field of boilers, for example, as in an exhaust heat recovery boiler disclosed in Patent Document 1 described below, a water supply method of converting (vaporizing) the water supply into water vapor after previously heating (preheating) the water supply using an exhaust gas (hot gas) of a gas turbine is performed. That is, the water supply method includes heating (pre-heating) the water supply with the combustion exhaust gas using a heat exchanger. In the boiler system using such a water supply method, the water supply after the preheating is converted into the water vapor in a boiler main body.