Flat detector and medium detector

A medium detector comprising:

• a transport unit that transports a medium in which a magnetic material is contained, the medium having a shape of a sheet;
• a flat detector that detects a signal from the magnetic material contained in the medium;
• and
• a sorting unit that switches between transportation paths for the medium that has passed through the flat detector in accordance with a result of detection of the signal by the flat detector,
• wherein the flat detector comprises:
• a first flat member and a second flat member both made of a nonmagnetic nonmetal, the first flat member and the second flat member being disposed to face each other with a space therebetween for transporting the medium;
• a first conductive wiring that generates an alternating magnetic field, the first conductive wiring being disposed on a surface of the first flat member facing toward the second flat member;
• a first layer made of a nonmagnetic metal and disposed at least on a surface of the first flat member opposite to the surface on which the first conductive wiring is disposed;
• a second conductive wiring that detects a signal generated by magnetization reversal of a magnetic material contained in the medium, the magnetization reversal being caused by the alternating magnetic field generated by the first conductive wiring, the second conductive wiring being disposed on a surface of the second flat member facing toward the first flat member;
• and
• a second layer made of a nonmagnetic metal and disposed at least on a surface of the second flat member opposite to the surface on which the second conductive wiring is disposed, wherein
• a thickness of the first flat member is set so that a strength of the alternating magnetic field at a contact surface of the first flat member and the first layer is 1 gauss or smaller.