To prevent excessive temperature rise of supply water in an economizer in a supply water preheating boiler preheating the supply water by the economizer 4.

In the supply water preheating boiler supplying water to a boiler body 6 via the economizer 4 and having an operation controller 8 controlling water supply to the boiler based on the boiler state, there are provided a water level detector 12 detecting a water level of the boiler, a preheated water temperature detector 5 detecting a temperature of preheated water taken out of the economizer 4, and a saturation temperature detector 7 detecting a saturation temperature of the boiler. The operation controller 8 controls to start water supply when the water level in the boiler detected by the water level detector 12 is lowered down to a water supply start water level, or when the preheated water temperature detected by the preheated water temperature detector 5 is increased up to a set temperature determined based on the saturation temperature, and to stop water supply when the water level in the boiler detected by the water level detector 12 is increased up to a water supply stop water level.