PURPOSE: To make it easy to monitor the liquid surface in a container by monitoring the attenuation of a diffused stress field which is propagated in a sound-wave propagation member.

CONSTITUTION: A transducer 18 which is coupled with the propagation member 16 for an ultrasonic wave, etc., transduces electric pulses from a pulse generator 22 into a stress wave and sends it out to the member 16. The stress wave is propagated in the member 16 and echoed to produce the diffused stress field. The diffused stress field is attenuated by the member 16 itself and liquid touching the member 16 and its attenuation quantity is affected by the area of the part that the liquid touches. Therefore, the transducer 20 detects the level of the diffused stress field and an amplifier 24 and an analyzer 26 measure the attenuation of the diffused stress field and output it to a display device 28 and a recorder 30. Consequently, the liquid surface in the container can be monitored.