PURPOSE: To enhance economy of once-through boiler operation by calculating an optimal humidity for preventing superheating of an evaporation pipe during normal operation of an once-through boiler and then controlling water supply and drain water from a steam separator using the optimal humidity as a control target value.

CONSTITUTION: A water supply regulation valve 5 and a drain water regulation valve 10 are regulated during normal operation to decrease the humidity of inlet steam of a steam separator 9 gradually such that at least one evaporation pipe 2 is brought into superheated state temporarily before returning back to normal operation. Based on the temperature and pressure of outlet steam having highest temperature (hereinafter referred to lowest moisture evaporation pipe) among superheated evaporation pipes 2, enthalpy of outlet steam of the lowermost moisture evaporation pipe 2 is determined. Subsequently, relation between the enthalpy of outlet steam of the lowermost moisture evaporation pipe 2 and the enthalpy of fluid at the inlet and outlet of a heating furnace 1 is determined and moisture check operation is conducted.