To facilitate the washing work of a boiler tank and the loading and unloading thereof by installing a weighing plate for mounting the boiler tank thereon which is energized in the rising direction by a spring and is freely ascendably and descendably arranged in the bottom part of a casing and a sensor for detecting the displacement of the weighing plate, and subjecting a solenoid valve to opening/closing control based on an output signal form the sensor.

City water is fed to a boiler tank 20, and is heated and vaporized in the boiler tank 20 to form steam. The steam is fed to a radiator 28 to turn it into distilled water, which is stored in a water storage tank. In the bottom part of a casing 4 of a distilled water vessel, a weighing plate 24 on whose upper surface the boiler tank 20 is mounted is freely ascendably and descendably arranged and is energized in the rising direction by a spring. The descending displacement of the weighing plate 24 on which the boiler tank 21 is mounted is detected by a sensor to detect the quantity of water in the boiler tank 20. Based on an output signal of the sensor, a solenoid valve installed in a line for feeding city water to the boiler tank 20 is subjected to opening/closing control.