PURPOSE: To produce an inexpensive drain valve generating no water leak even when scale gets jammed in the valve seat of the drain valve by suppressing the adhesion of the scale generated on a heating surface when steam is generated using water containing scale components such as tap water by discharging a part of scale component conc. water.

CONSTITUTION: In a boiling device boiling water containing a scale component, a temp.-sensitive valve 4 is provided in a drain route and only water lowered in temp. by the radiation in the drain route is discharged to control the drain amount of water. An overflow part 9 is set to the position almost same to the water level of a boiling tank 1 to make the pressure difference before and behind the valve extremely slight and the generation of a water leak is prevented even when scale gets jammed in a valve seat.