PURPOSE: To prevent a heating capability of water supplying of a water supplying and heating device from being decreased even if a drain of another water supplying and heating device in another system is rapidly increased by a method wherein when a state for supplementing a system part where the other system is separated, a set water level in the water supplying and heating device in the other system is decreased.

CONSTITUTION: A water supplying and heating device 3 has a water level controller for use in monitoring a water level in a drain 7. At a water level controller 8, the water level in the drain 7 is inputted to water level control circuits 8A, 8B and controls valves 9, 10 for getting a deviation between the inputted water level and the set water levels in water level setting devices 88A and 88B. Then, in the case that a heater cut state occurs, the set water level set lower than the set water levels of the water level setting devices 88A and 88B are inputted to the water level control circuits 8A and 8B, With such an arrangement as above, it is possible to improve a stability in output of the plant without decreasing a water supplying and heating efficiency.