A piezoelectric detector for detecting a physical quantity comprising, a piezoelectric device having a first electrode and a second electrode for modulating a carrier A.C., a excitation power source for supplying the carrier A.C. to the first electrode so as to excite the piezoelectric device, a weight mechanically coupled to the piezoelectric device so as to apply the physical quantity to the piezoelectric device, an A.C. amplifier for amplifying a modulated output from the second electrode, and a demodulator for demodulating an amplified output from the A.C. amplifier.
The piezoelectric detector measures the physical quantity having the very low frequency range including D.C. component.