A potential sensor including first and second detection electrodes opposed to an object of which a potential is to be measured, and a movable shutter so positioned between the detection electrodes and the potential-measured object with gaps thereto; wherein the movable shutter can assume a first state and a second state, the first detection electrode is exposed to the potential-measured object wider when the movable shutter assumes the first state than when the movable shutter assumes the second state, and the second detection electrode is exposed to the potential-measured object narrower when the movable shutter assumes the first state than when the movable shutter assumes the second state.