A flat detector includes a first flat member and a second flat member both made of a nonmagnetic nonmetal, 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, a second conductive wiring that detects a signal generated by magnetization reversal of a magnetic material, 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.