PURPOSE: To detect whether liquid is present or not and to generate an alarm by detecting the electrostatic capacity between a couple of electrodes provided in a tank.

CONSTITUTION: A main body 19 which incorporates a liquid level alarm circuit 17 is fitted to the upper part of the tank 18 and the electrodes 8 and 9 are put in the liquid 21 in the tank across a seal material 20. In this case, the electrodes 8 and 9 are covered with an anticorrosive material such as glass so as to prevent contamination, corrosion, etc. Then, the switching circuit of the alarm circuit 17 is turned on or off by variation of the current between source voltages V1 and V2 to the electrodes 8 and 9. Consequently, the switching element in the alarm 17 is turned off and a current I flows, so that the upper end voltage of a resistance 24 drops. Such an output W is detected by a detecting circuit to easily receive an alarm signal at distance.