PURPOSE: To detect a liquid level in a container through which it can not be seen by vertically moving either or both of a signal wave generator and pickup unit along the flank of the container and then by detecting a change in level of a signal wave propagating to the pickup unit.

CONSTITUTION: When a signal wave generator is installed at position A on oil case flank 1 and a pickup unit is connected to flank 2 at position B, the pickup unit received a signal wave propagating in the order of flank 1, oil 3 and flank 2 at position B and that in the order of flank 1, oil 3, air 4 and flank 2 at position B'. Here, a liquid differs from air in the propagation speed and density of an acoustic wave of signal wave of vibration, causing the reflection, etc., of the signal wave. Namely, there is a difference in level of the signal wave between above-mentioned sections A-B and A-B', so those levels are detected to find the generated position of a change in level, so that the liquid level can be measured rapidly and accurately.