A proximity sensor includes a sensor cable that includes a first electrode wire and a second electrode wire arranged parallel to each other, an insulation covering both the first electrode wire and the second electrode wire, and a shield partially covering a surface of the insulation so as to form an opening, the first electrode wire and the second electrode wire being arranged to have different distances to the opening, and a detector circuit that includes a first capacitance detecting portion for detecting a first capacitance to be detected by the first electrode wire, a second capacitance detecting portion for detecting a second capacitance to be detected by the second electrode wire, and a differential output portion for outputting a difference between the first capacitance and the second capacitance.