PURPOSE: To make special parts such as a chopper electrode to be provided between the opening of a shield member and a measuring electrode unnecessary and to reduce the thickness of the whole sensor and to reduce the number of parts by vibrating directly the opening of shield members.

CONSTITUTION: Electric line of force E of which potential is to be measured are made incident into a shield case 10 through an opening 12, received by the measuring electrode 11 and converted into a signal indicating the surface potential by a potential detecting circuit 15. The shield member 10C, 10D are vibrated by the action of a piezoelectric element 13 energized by a driving circuit 14, the opening part 12 is vibrated as shown by the arrow A and the area of the opening 12 is periodically changed. The number of electric line of force received by the measuring electrode 11 is also changed similarly and converted into an AC signal. The potential detecting circuit 15 processes these AC signals and outputs a signal indicating the measuring potential, so that the thickness of the sensor and the number of parts are reduced.