PURPOSE: To detect the residue of a fluid body accurately, by providing a piezo-electric vibrator with a feedback electrode so that the oscillation frequency of an oscillation circuit shifts from an oscillation stoppage area or a harmonic oscillation area to an audible oscillation area with a specified residue of the fluid body.

CONSTITUTION: A mounting case 1 has a step 3 formed to permit the mounting a circular buffer support 2 comprising rubber or the like thereon. A vibrator 4 is so arranged that the circumferential end thereof 4 is mounted against the support 2 to be allows to vibrate freely in the cavity 5 of the case 1. For oscillation frequency of an oscillation circuit, a circuit constant is properly set so that the circuit can oscillates at an audible frequency in such a manner that a fluid body such as liquid remains slightly on the vibrator 4. Therefore, the state about 0.5W1cm in the residue of the fluid body is established as shifting point. When the residue of the fluid body is below the shifting point, the oscillation is performed at the audible oscillation area high in the oscillation output level while at the oscillation stoppage area or the harmonic oscillation area when it is above the shifting point.