PURPOSE: To detect liquid surface level with a simple, inexpensive structure, by providing two coils of the same type but with opposite windings, above and below with respect to the liquid surface, in a probe placed inside a metal liquid container.

CONSTITUTION: When the liquid surface lies at a level lower than coil 12A, since the ambient conditions of coils 12A and 12B are equal, their inductances are equal, and the resultant inductance of coils 12A and 12B are zero, so that logic circuit 20 displays on display 21 that the liquid surface level is lower than the position of coil 12A. When the liquid surface lies at the midpoint between coils 12A and 12B, the resultant inductance of coils 12A and 12B shows a large value. When the liquid surface rises still higher and lies at a level higher than coil 12B, the inductances of coils 12A and 12B become small and equal. Logic circuit 20 discriminates these and shows the liquid surface level on display 21.