To obtain a small and lightweight sound pressure detector having high detection sensitivity and excellent in high speed response which can be driven with low power consumption by detecting a sound pressure propagating through a gas and representing the detected sound pressure as an electric signal.

When an input electric signal is applied to an input blind-like electrode 2, a leakage elastic wave is excited in a piezoelectric substrate 1. The leakage elastic wave propagates, as a longitudinal wave, through a liquid in a liquid chamber 4 before being reflected on an elastic film 7. The reflected longitudinal wave is detected, as a delay electric signal, by a blind-like electrode 3. When the elastic film 7 senses an external sound pressure, the elastic film 7 vibrates mechanically to cause variation of the delay electric signal. Variation of the delay electric signal is detected by a signal analyzing means 8 and displayed on a display screen 9.