PURPOSE: To make 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 measuring electrode.

CONSTITUTION: Electric line of force E of which potential is to be measured are made incident into a shield case 10 through the opening 13 of the shield case 10, received by the measuring electrode 12 and converted into a signal indicating the surface potential by a potential detecting circuit 20B. The measuring electrode 12 is vibrated by a tuning fork vibrating chip 11 resonantly oscillated by the action of a driving element 14 energized by a driving circuit 20A as shown by the arrow A and periodically changed between the minimum area and the maximum area. The number of electric line of force incident through the opening 18 is changed in accordance with the periodical change and converted into an AC signal. The potential detecting circuit 20B processes the AC signal and outputs a signal indicating the measuring potential. Consequently, the parts such as a chopper are made unnecessary, the thickness of the whole sensor is reduced to reduce the number of parts and always precise measuring result is obtained.