PURPOSE: To prevent priming and carry-over by regulating the opening of a proportional control valve of a fuel supply passage in response to a signal of a moisture sensor for measuring the wetness of steam of a steam tube coupled from the steam outlet of a boiler body to a steam using unit.

CONSTITUTION: A proportional control valve 11 is disposed in a fuel supply passage for supplying fuel to a burner 9, and a moisture sensor 12 for measuring wetness of steam is mounted at a steam tube 4. If priming or carry-over starts to occur, the wetness of steam fed from a steam outlet of a boiler 2 rises. Accordingly, a controller 13 controls the valve 11 in a direction for opening the opening of the valve 11 by a signal from the sensor 12 to enhance heating capacity of the burner 9. Thus, the priming, carry-over is prevented.