PURPOSE: To prevent the flow of unnecessary low electric current to the line of a warning lamp by connecting an electric power source and a warming means via a thermistor which is installed in the warning level position in a tank and a switch which is operated by the displacement of a float.

CONSTITUTION: When fuel is sufficiently contained in a fuel tank and the level thereof is above the level slightly upper than the warning level, an arm 8 is in a turning position and a switch 17 is held opened. Therefore, the line connecting to a warning lamp is held interrupted. When the level lowers below the warning level on decreasing of the fuel, the switch 17 is closed by the arm 8, and the resistance value of a thermistor 3 drops down to a required value, thereby lighting the warning lamp.