PURPOSE: To prevent the fuel leakage at the time of a failure of a vaporizing cylinder by providing a variable resistor type temperature sensor in a drain passage to detect the drain flow rate so that the fuel supply can automatically be shut off at the same time as the drain outflow.

CONSTITUTION: The drain is led from a drain port 12 of the outer framework 11 through a drain hose 13 to a drain cup 14. During such a passage, it wets a variable resistor type temperature sensor TS, and is vaporized by the heat generated in the sensor itself, at it cools the sensor TS by the vaporization heat. Because of this, the temperature of the sensor TS is reduced to cause its resistor capacity to change. At this time, the change in the terminal voltage of the sensor TS is A/D-transformed, and a shutoff signal is outputted from a CPU18 to shut off a pump 1, a solenoid valve 2 and a vaporizing cylinder motor 5. In this manner, when a drain occurs, the fuel supply is immediately stopped so that the leakage of a large quantity of fuel out of the unit may be prevented.