PURPOSE: To detect safely and surely arrival at a prescribed liquid level with good responsiveness for a liquid such as gasoline etc. also, by detecting a sudden change of an oscillation output by an inductance coil acting correspondently to a magnetic piece having a Curie point.

CONSTITUTION: A temperature of a temperature sensor 3 formed by a magnetic piece conducting an electric current and having a Curie point at the room temperature, is lowered below a reference temperature by depriving heat by a liquid 2. When a liquid level of the liquid 2 is lowered and is reached a prescribed liquid level and then, the temperature of the sensor 3 is attained to the reference temperature, magnetic permeability of the magnetic piece of the sensor 3 is changed suddenly and inductance of an inductance coil of an oscillation circuit 4 is varied. Then, frequency of an oscillation output or a sudden change of amplitude of the circuit 4, is detected. By this constitution, arrival at the prescribed liquid level is dectected safely and surely with good responsiveness for a liquid such as gasoline etc. also.