PURPOSE: To provide a device which can accurately detect the level of a liquid under an atmosphere of high presure and temperature, by housing a heater and a temperature detection means in a detection pipe.

CONSTITUTION: A thin detection pipe 40 has a construction to resist service pressure and temperature. A heater 41 and a temperature detection means 42 are provided in the pipe 40. Lead wires 46, 47 are retained by an electric insulating substance 45 filled in the pipe 40. The pipe is attached to a lower part of a vessel 10 containing a liquid to br measured. The heater 41 is connected to a power source 8. The temperature detection means 42 made of a thermocouple is connected to a controller 5. When the level of the liquid in the vessel 10 is high, the heat of the heater 41 is transferred into the liquid and the temperature around the heater is constant. When the level of the liquid falls as shown at 3, the temperature around the heater rises. At that time, the rise in the temperature is detected by the means 42 and the liquid is supplied by an instruction from the controller 5 to raise the level.