PURPOSE: To obtain the detection characteristic with less disturbance by increasing the impedance of a parallel circuit comprising a coil and a capacitive element in the three-times mode frequency of a piezoelectric resonator to reduce the effect of a phase characteristic onto a main detection characteristic.

CONSTITUTION: A capacitive element such as a capacitor 45 is connected in parallel with a coil 43 being a component of a phase shift circuit 4. Then the self-resonance frequency of the parallel circuit comprising the coil 43 and the element 45 is adjusted freely based on the capacitance of the element 45 to increase the parallel circuit impedance at the 3-times mode frequency of a piezoelectric resonator 42 than the impedance of the coil 43 itself at the frequency Thus, the damping of the 3-times mode of the resonator 42 is strengthened and the effect of the phase characteristic onto the main detection characteristic is reduced and the excellent detection characteristic with less disturbance is obtained.