PURPOSE: To make it possible to effect water level control at two positions with two water level sensors and make it possible to supply two kinds of steam with different dryness by means of a change-over switch by providing both A and B circuits which each have a specified function and making both circuits capable of being mutually changed over freely.

CONSTITUTION: In a control device 6, a control circuit A outputs OFF signal to a water supply pump 4 by the water level detection signal from a water level sensor 2. At a set time t1 after the signal that the water level detection can not be made was issued from the water level sensor, the control circuit A issues ON signal to the water supply pump 4. The circuit B issues On signal to the water supply pump 4 by the signal from the water level sensor 2 that the water level detection can not be made, and at a set time t2 after the output the circuit B issues OFF signal to the water supply pump 4. A change-over switch 7 mutually changes over the circuit A and circuit B. A heat source shut-off circuit C shuts off a heat source 5 by the signal from a low water level sensor 3 that the water level detection can not be made. Also a boiler 1, water level sensor 2, low water level sensor 3, water supply pump 4, heat source 5, control device 6, and change-over switch 7 are provided.