A paper-sheet-thickness detecting sensor of the present invention includes: an oscillator that outputs high-frequency signals with frequencies corresponding to positions in a horizontal direction that is orthogonal to a direction in which a paper sheet is transported; a signal electrode to which the high-frequency signals output by the oscillator are applied; a plurality of detection electrodes that face the signal electrode across a transportation path for the paper sheet and that individually detect detection signals that correspond to the positions; a plurality of resonance circuits that each have one end connected to one end of each of the plurality of detection electrodes and that have resonance frequencies equal to the frequencies allocated to the positions; and a broadband amplifier to which another end of each of the plurality of resonance circuits is connected and that outputs outputs of the plurality of resonance circuits as one piece of sensor output information.