PURPOSE: To safely and simply detect reduced electrolyte by arranging edges of a pair of optical fibers in a facing spaced position so as to slope an optic axis to a liquid level in a liquid level to be detected.

CONSTITUTION: A supporter 6 is set in a cover 2 of a storage battery, and a cut-off portion 7 is installed in the lower part of the supporter, in which an electrolyte level is detected. The first and second optical fibers 8 and 9 are placed in the cut-off portion so as to face and slope. An LED11 and a photodiode 12 in a detecting portion 10 are arranged in the other ends of photo fibers 8 and 9 respectively. When the liquid level of an electrolyte 1 is high, light reaches the photodiode 12 through the electrolyte 1, so a lamp 13 does not go on and off. When the liquid level is low, light refracts on the interface of the air layer and the liquid level, and the lamp 13 goes on and off. Therefore, the liquid level is simply and safely detected.