A detecting device includes a first coil, a third coil, a second coil, and a fourth coil. The first coil generates a first magnetic field on a to-be-measured object. The third coil generates a third magnetic field under the to-be measured object. The second coil generates a second magnetic field. After the fourth coil receives the second magnetic field, a voltage is induced. The induced voltage is amplified by an amplify circuit to drive the third coil. The directions of the currents generated by the first coil and the third coil, respectively, are the same.