It is pointed out that responsivity can be improved by mechanical resonances. The enhancement factor of responsivity of a trigricyne sulphate (TGS) pyroelectric detector is measured as 40 ∼ 50 at the mechanical resonances whose modes are assigned to dilational modes of a rectangular-plate vibrator. Also, a pyroultrasonic parametric amplifier is demonstrated, which can show further improvement of responsivity of the pyroelectric detector by amplifying the strain that accompanies pyroelectricity. Gain and an equivalent circuit are derived using a simplified parametrically coupled multimode vibrator model. A pyroultrasonic parametric amplifier using lead zirconate titanate (PZT) ceramic is demonstrated, and an improvement of about 5 dB is shown. Obtaining the gain stability is one of the serious problems of a pyroultrasonic parametric amplifier.