PURPOSE: To know the presence or absence and quantity of q liquid contained in a vessel which cannot be seen from the outside by sticking piezoelectric ceramics to a diaphragm of a susceptible part fitted to a vessel wall, exciting the diaphgram by applying an electrical pulse signal, detecting an attenuating state as quantity of electricity, and knowing a liquid level position.

CONSTITUTION: An output of an oscillator 14 for oscillating periodically a pulse signal is boosted by a boosting transformer 15 and is applied to piezoelectric ceramics 7 of a susceptible part 3 by conductors 12, 13. By this signal, the piezoelectric ceramics 7 generate displacement mechanically, are driven, and a diaphragm 6 is vibrated. In case when the quantity of a liquid is small or there is no liquid and no liquid contacts with the diaphragm 6, amplitude of the attenuating vibration is large, and the vibrating time is also long, but it case when liquid pressure operates on the diaphragm 6, the amplitude is small and the vibrating time also becomes short. The vibrating stage of the diaphragm 6 is fetched as an electric output of the piezoelectric ceramics 7 which are vibrated with the diaphragm 6 and are deformed minutely, and is inputted to an amplifier. An output of the piezoelectric ceramics 7, amplified by the amplifier 16 is rectified by a rectifier 18, and a total area of a rectifying waveform is calculated by an integrator 19.