and 3 are made of non conductive synthetic resin such as polyacetals or the like the lower and upper casing components 2 and 3 have cylindrical circumferential wall sections 2a and 3a mating at free end edges thereof an annular and cross sectionally square clearance is defined between the mating edges of the lower and upper casing components 2 and 3 to receive therein an annular sensor support member 4 the sensor support member 4 is made of an elastic material such as polyurethane resin the sensor support member 4 engages with the edge of the oscillable piezoelectric sensor 1 for oscillably supporting the latter within the internal space of the sensor casing the lower and upper casing components 2 and 3 are sealed and rigidly bonded or are welded in such a way that the enclosed internal space of the sensor casing is completely sealed a circuit board 5 on which various electric and or electronic circuit components 6 such as a filter an amplifier and so forth are mounted the circuit board 5 lies substantially parallel to the bottom 2b of the lower casing component 2 and is rigidly secured thereonto by means of fastening screws 5a the circuit board 5 and the circuit components 6 are buried in a potting resin layer 7 which is filled after rigidly fastening the circuit board 5 onto the bottom 2b of the lower casing component 2 the circuit board 5 is connected to an external circuit via a lead wire 8 which extends through a grommet 8a which is used as a seal and which is engaged on the outer periphery of the circumferential wall section 2a of the lower casing component 2 the piezoelectric sensor 1 comprises a thin disc shaped metal.