PURPOSE: To make it possible to detect the specific gravity due the level of electrolyte and give an alarm if necessary by installing a float which floats and sinks in the electrolyte in a battery and the sensor for detecting the position of the float.

CONSTITUTION: When the level Wo of electrolyte lowers below the level WL at which alarm is to be given, a float 6 also lower, following said level, to face a reed switch 5 and a magnet 7. Then the contact of the reed switch 5 is closed, duly subjecting the condenser C in a delaying circuit 22 to electric discharge through a resistor R2, and a transistor T2 is turned OFF after the lapse of a time constant (R2×C). When a transistor T1 is turned OFF, the base potential of the transistor T2 rises, and an alarm lamp LP comes ON when the specific gravity is reduced. Even if the level of electrolyte 9 is not varied, the float 6 sinks and is kept at the stepped part 10. In an other word, the contact of the reed switch 5 is closed by a magnet 7, and the alarm lamp LP comes ON.