PURPOSE: To simplify operation and to make the device compact, by providing a light emitting element and a light receiving element at the corresponding measuring-level positions at the outside of a transparent tubular container with a closed curve, respectively.

CONSTITUTION: A transparent tubular container 4 forms a closed curve and transmits visible light or infrared rays. At the corresponding position across the container 4, a light emitting device 2 and a light receiving device 3 are provided. The devices 2 and 3 are arranged so that a light path connecting the light emitting element and the light receiving element does not pass the center of the cross section of the container 4. When a transparent liquid is present in the container 4 so that the surface is higher than the light path, the light beam from the light emitting element does not reach the light receiving element. Only when the surface of the transparent liquid is lowered less than the height of the light path, the light beam from the light emitting element reaches the light receiving element. Then, the liquid level is detected, and a warning device 10 is actuated by the detected output.