PURPOSE: To enable to detect electric field strength with high accuracy by a method wherein a sound conductive tube made of an insulator is installed between a vibrating plate coupled with a piezo-electric element and a microphone.

CONSTITUTION: When a piezo-electric element 4 is inserted into a place to be place to be detected, the piezo-electric element 4 is distorted in proportion to an electric field strength by means of an inverse piezo-electric effect and oscillates with a given frequency to vibrate a vibrating plate 7. With this, an acoustic wave is generated from the vibrating plate 7, passes through a sound conductive tube 1 and reaches a microphone 10. Here, it is converted into an electronic signal and is supplied to an indicator 12 via an amplifier 11, thus, the field strength of the place to be detected being displayed.