PURPOSE: To simplify the equipment by actuating an alarming device detecting changes in the amount of light received with a light detector as soon as an optical fiber connecting a light transmitter and the light detector comes into a liquid to a fixed length.

CONSTITUTION: When an optical fiber 3 which an exposed protective layer comes below the liquid level by the lengh l of the bent portion, light entering the light detector 2 decreases after propagated through the optical fiber 3 from a light transmitter 1. The detector 2 detects the change in the quantity of light to actuate a alarm transmitter 6. Accordingly, the liquid level is detected with a simple equipment to transmit a alarm.